I will give a Lutheran response to this question. It is a position that is reflected in the common European legal approach to abortion. It is one that is different than what Roe v. Wade decided, in that it restricts abortion after 18-24 weeks of a fetus developing in the womb.

In the Lutheran tradition of doing ethics there is a distinction to be made between two kingdoms.

In the theology of the two kingdoms, the kingdom of the right would include all those things that are related to those that impact the church directly. The basis for reasoning in the kingdom of the right is how to interpret Scripture in relationship to the commandment, "You shall not kill."

The traditional Christian view is that God creates souls ex nihilo and supplies them to a developing person at conception, or during a further time of fetal development in the womb.

Thomas Aquinas asserted that the male receives his "rational soul" forty days after conception. However, following Aristotle, Aquinas believed the biological female does not receive a soul until eighty to ninety days (Summa Theologica, Pt. I, quest. 75, art. I; cf. quest. 76, art. iii ad3; quest. 118, art. ii ad 2.).

In support of the mediate animation theory of Aquinas is how newly fertilized human ovum can be forced to divide and separate, thereby producing identical twins. For those holding to ensoulment taking place at conception, those cases would appear to negate the scholastic tradition that the soul as a "rational" or "spiritual" entity is indivisible (simplex).

Among conservative Protestants, this mediate animation view was held by some theologians at the Evangelical Symposium on the Control of Human Reproduction held in Portsmith, New Hampshire as far back as 1968. For example, Kenneth Kantzer at the event had reservations as to a fetus possessing a soul from the moment of conception. See J.W. Montgomery's Slaughter of the Innocents: Abortion, Birth Control & Divorce in Light of Science, Law & Theology.

Others have argued that modern embryological knowledge should push Christians to assert that the soul is supplied by God when conception occurs.

The safest hypothesis is to rest on the clear words of Scripture. The clearest example of determining the answer to this question is the joy filled leaping of John the Baptist in his mother's womb (Luke 1:41). Unless this is merely a poetical image, it appears to indicate personhood takes place in the womb sometime prior to birth.

The kingdom of the left, in Lutheran theology, is seen as being illumined by Scripture (kingdom of the right) to a common reasoning that is shared with those who have no religious beliefs in the public square. In that sense, the pro life position is not strictly a religious position, it is a human rights position.

Here the leaping of John the Baptist is congruent with what science has revealed about a developing fetus having brain waves, heart beats and a type of respiratory activity through an umbilical cord (somewhat analogous to a cord that a space walker is attached to). For example, at 24 to 27 weeks of pregnancy, the characteristic human-specific brain-wave pattern emerges in the fetus’s brain. Indeed, the disappearance of this is part of the legal standard for human death. By symmetry, its appearance should logically mark the beginning of human life.

The big question is whether the freedom to reproduce can justify infanticide in utero, other than self defense, especially during the 3rd trimester of pregnancy? If brain waves & heart beats define a person’s right to human and civil rights, than abortion (especially 3rd trimester) should not be treated as a form of birth control.

On a sliding scale of epistemology relevant to the crafting of laws against abortion, there is a high likelihood (i.e. a standard of proof with a moral certainty beyond reasonable doubt, commonly used in criminal law) that a person with a soul exists in utero during the 3rd trimester of pregnancy. The safest hypothesis than is to reject abortion as a form of birth control, especially during the last trimester of pregnancy.

This is in contrast to the view of Roe v. Wade. Justice Blackmun, speaking for the Court stated (emphasis added):

We need not resolve the difficult question of when life begins. When those trained in the respective disciplines of medicine, philosophy, and theology are unable to arrive at any consensus, the judiciary, at this point in the development of man's knowledge, is not in a position to speculate as to the answer. (Roe v. Wade, 93 S Ct. 705)

Given the view above, the focus of the church can be to teach the controversy. Ideally, this should be done in way that does not obscure the Gospel. Christians should appeal to common reasoning, by laying out persuasive arguments for a developing infant in utero being given the full human & civil rights protection that those outside of the womb currently have.

Teaching the controversy, while living peacefully in a society that condones infanticide in utero, is a return to the 2nd & 3rd century era. For example, the Christian lawyer Minucius Felix in that era declared:

I see that you at one time expose your begotten children to wild beasts and to birds; at another, that you crush them when strangled with a miserable kind of death. There are some women who, by drinking medical preparations, extinguish the source of the future man in their very bowels, and thus commit a parricide (i.e. infanticide) before they bring forth. (Chapter 30)

We need not resolve the difficult question of when life begins. When those trained in the respective disciplines of medicine, philosophy, and theology are unable to arrive at any consensus, the judiciary, at this point in the development of man's knowledge, is not in a position to speculate as to the answer. (Roe v. Wade, 93 S Ct. 705)

This statement points up the very common conflation of scientific fact with moral, philosophical, and legal opinion. Below are a few excerpts from an excellent article by Michael Egnor, Senior Fellow, Center for Natural & Artificial Intelligence. Michael R. Egnor, MD, is a Professor of Neurosurgery and Pediatrics at State University of New York, Stony Brook, has served as the Director of Pediatric Neurosurgery and an award-winning brain surgeon.

His well presented position is that from a purely scientific, biological standpoint the only valid option is that a separate, individual human life begins at fertilization. It is the job of moral, philosophical, and legal opinion to then decide if and when it is acceptable for an individual human being to deliberately take the life of a separate, individual human being. His contention is that so much of the rancor and frustration in the abortion debate comes from disregarding this basic scientific fact or conflating it with morality and metaphysics. Although I have quoted extensively from the article, the entirety is worth perusal.

... much of the rancor and division in the abortion debate stems, I think, from conflation of scientific and moral issues. They are not the same. Properly formed moral views depend on correct scientific understanding. If we don’t know what a fetus is, scientifically, we are hampered in making sound moral judgments about its protection.

We all agree that life begins at or after union of the sperm and egg. Fertilization marks the earliest moment in human development that human life might begin. Many other times after fertilization have been proposed — the first appearance of nervous tissue, or heartbeat, or brain waves, or quickening, or the ability to feel pain, or viability outside of the womb, or birth, or rationality, etc. At first, it seems a hopeless conundrum: there seem to be so many possible moments at which life begins that science is stymied. Perhaps science can’t tell us when life begins. But science most certainly can tell us, and does tell us.

Assuming that human life does not begin or may not begin at the moment of fertilization only leaves so many possibilities as to what the "pre-life" tissue may be:

So the critical scientific issue at the heart of the question “when does human life begin” is: What scientific description of the tissue (let’s call it the “fetus” for brevity) before human life begins makes sense biologically? Consider the options: 1)The fetus is a part of the mother’s body, 2)The fetus is not part of the mother’s body, but is an individual of another species, 3)The fetus is not any kind of living thing — it’s just a clump of biological molecules undergoing chemical reactions.

1. The fetus is a part of the mother’s body. if the fetus is a part of the mother’s body, then all pregnant women are chromosomal mosaics. That is, they are organisms that have two sets of genomes. Chromosome mosaicism is a rare disorder and is not synonymous with pregnancy. There is no such thing as “transient chromosomal mosaicism.” Furthermore, if the fetus is a part of the mother’s body, then half of pregnant women are hermaphrodites — i.e., they contain both male and female tissues. Needless to say, “transient gestational hermaphroditism” is not a recognized medical disorder. Furthermore, if a new human life begins by a piece of the mother’s body becoming a new organism, then human beings reproduce by budding. Budding is a form of asexual reproduction used by some species of worms, sponges, corals, and microorganisms, but it is not a means of human reproduction. There is no biological sense to be made of the claim that “the fetus is part of the mother’s body.” The claim leads to scientific implications that are nonsense.

2)The fetus is not part of the mother’s body but is an individual of another species. If the fetus is an individual member of another species, then pregnancy is by far the most common parasitic disease among humans. What’s more, the transition in each pregnancy from a non-human parasite to a new human being is speciation — the evolution of a new species, “Homo fetus” to Homo sapiens — occurring with each pregnancy. This is, of course, scientific nonsense.

3. The fetus is not any kind of living thing — it’s just a clump of biological molecules undergoing chemical reactions. If the fetus is not really living at all, then each pregnancy is a new origin-of-life event. This is also scientific nonsense.

So the biological answer to the question “When does human life begin?” is easy to answer from a scientific perspective: human life begins at the moment the sperm fertilizes the egg. This is not merely common sense and the claim of pro-life advocates, it is clear and straightforward science, as settled as heliocentrism and the existence of atoms. All cogent moral reasoning about abortion must begin with the understanding that human life begins at fertilization. There are no other scientific options.

There is a simple scientific answer to the basic question at the heart of the abortion debate. Whatever a “person” is, a human zygote is most certainly a human being. The term “person” is a moral and legal category, not a scientific category, and it is a category open to moral discussion and debate. But “human being” is a scientific term, and it is not open to debate. The science is settled. Human life begins at fertilization, and cogent moral reasoning about abortion must begin with that scientific fact.

The idea that human life begins at conception/fertalization, then, is not an idea pulled by Christians from the Bible or by any religion from their religious texts...it is the only valid scientific option. Christian pro-life advocates can take great encouragement that their sacred text harmonizes so perfectly with established science in this area.

The baseline fact is that a human zygote is a human being. Let all sides of the debate begin with that and make their moral, legal, and philosophical case as to the circumstances under which one human being may take the life of another.

The real distinction is between "human" and "person".

Purely atheistic science says that becoming a human (i.e. genetically Homo sapiens) occurs at conception.
The concept of "personhood" is strictly used for religious or legal/political purposes.

By definition, killing a fœtus is killing a human.

Christians (and most other religions) interpret "Thou shall not kill." as meaning not to murder another human being.

So, using objective scientific definitions, killing a fœtus clearly violates this rule.
No further justification is needed for an anti-abortion stance.

Politically though, the term "person" is used rather than "human".

This allows arbitrary definitions of what is or is not considered to be a person under the law.

It is those that support abortion-on-demand that need to justify their stance.
Lacking any objective scientific definition of what a person is, how do they justify their arbitrary choice of when a human fœtus becomes a legal person?

Exclusively religious arguments

There are three arguments that spring to mind as being at least completely religious arguments (if not exclusively Christian ones);

1. Ensoulment. When does the soul enter the body? This is going to be the decisive moment, for people of many faiths, at which the foetus becomes a living human.
2. Original sin. This follows from ensoulment, in as much as The soul is marred by original sin from the moment it was created, so if the foetus has already got a soul, then many would hold that it is not innocent and must be baptised to enter heaven. This one is specific to Christianity (or at least, as best I know, do say if you know of another faith with similar belief).
3. Prohibition on birth control. Should some point of view hold that the foetus does not yet have a soul, it may still condemn abortion as a form of birth control. (Admittedly, I suspect this position is unusual, but it's not untenable).

Below I will try and get into these three points of view in a bit more depth;

1. Ensoulment

There is a lovely discussion on the history of beliefs about Ensoulment on Wikipedia. I summarise it here as; The exact day that the soul enters the foetus varies between denominations; things range from right at the moment of conception (e.g. baptists), to not until the baby is born (Pope Innocent XI). Some denominations don't even have concrete dogma on the matter (e.g. new advent catholisism).

It's then a natural follow on argument to say that "Thou shalt not kill" applies equally to a foetus once it has a soul. Interestingly, the immorality of killing is more complicated than absolute in many denominations. Relatively few denominations are completely pacifist. So there will be further nuance on why and when a foetus with soul is to be saved over the pregnant woman with soul. There are probably many answers to that dilemma, but the next point covers at least one of them.

2. Original sin

If the pregnant woman has been baptised, should she die, she may go to heaven. The foetus is not so lucky; if we follow Thaschus Caecilius Cyprianus, it is "born has not sinned at all, except that carnally born according to Adam, he has contracted the contagion of the first death from the first nativity." So it needs to be baptised; John, verse 5 "Unless one is born of water and of the Spirit, he cannot enter the kingdom of God."

With these conditions, getting the foetus to term, when it can be baptised, becomes the overwhelming concern. Even over the life of the one who carries it.

Interesting note, while researching this answer, I came across a Jewish take on ensoulment and abortion, namely that the whole thing is a red-herring. This follows from the lack of original sin in Judaism. That doesn't mean that all Jewish people approve of abortion, there are reasons to disprove aside from ensoulment/original sin, some of which are entirely perpendicular to religious arguments. However it is noted that almost all branches of Judaism will favour saving the mother over the foetus, which follows rationally from the belief of the foetus being an innocent soul.

3. Prohibition on birth control

Let us take the position of Pope Innocent XI; that the "fetus (as long as it is in the uterus) lacks a rational soul and begins to first have one when it is born". Does it then follow that abortion is harmless?

Perhaps not, if birth control is wrong. For example, in the Amish denomination, any form of family planning is immoral. I don't have any sources either way on Amish belief about ensoulment, so it may well be that the birth control argument is superfluous here. Actually, I'm not aware of any Christian denomination that has a prohibition on abortion only because of issues of birth control, although we have (I think) a plausible argument here. Can anyone name one?

Closing note

I felt that many of the answers here bring up points that draw on a moral framework that is rather broader than religion. For example, if abortion was truly about consciousness, or pain perception, then surely at least one Christian denomination would require veganism which is normally motivated by those same tenets? While many Christians are vegan, a denomination prohibiting meat and dairy is something I have never heard of.

So I wanted to contemplate the exclusively religious aspects of this question, including those I'm not adherent to. If you feel I have misrepresented any of those quoted here, please do say. I'm keen to learn about other angles.

I don't really see the need for long complicated arguments. Anti-abortion advocates (whether Christian or otherwise) are arguing from a single axiom:

An unborn baby is a person

That's basically it. If you apply any pro-abortion arguments but replace "unborn baby" with "baby" it would seem incredibly barbaric. "Can I kill my baby if it is a result of rape" - clearly the answer is no.

The question is when someone becomes a person. Different groups have different answers informed (or misinformed) from religious, medical and moral arguments commonly but not exclusively including:

• At birth: there is a fundamental change once the baby is delivered
• At the point the child would possibly/probably survive if delivered (this informs a lot of rules such as 24 week limits based on typical survivability)
• At the point a fetal heartbeat is detectable
• From conception

Whichever decision point you use, terminating after that point is logically to kill a child and when that's how one group sees it, there can't be too much reasonable debate.

The biblical arguments center on God talking about the fetus in the mother's womb as a known person. It starts way back in Genesis.

And the LORD said unto her, Two nations are in thy womb, and two manner of people shall be separated from thy bowels; and the one people shall be stronger than the other people; and the elder shall serve the younger. Gen 25:23

But he said unto me, Behold, thou shalt conceive, and bear a son; and now drink no wine nor strong drink, neither eat any unclean thing: for the child shall be a Nazarite to God from the womb to the day of his death. Judges 13:7

Did not he that made me in the womb make him? and did not one fashion us in the womb? Job 31:15

I was cast upon thee from the womb: thou art my God from my mother's belly. Psalm 22:10

Thus saith the LORD that made thee, and formed thee from the womb, which will help thee; Fear not, O Jacob, my servant; and thou, Jesurun, whom I have chosen. Isa 44:2

Simply look to science:

• Hippocrates was a physician noted for separating medicine and religion, believing that disease has physical origins, not because of demons or divine punishment. Despite being as close to an atheistic or agnostic view as one could achieve, his Hippocratic Oath prohibited both euthanasia and abortion for nearly two and a half millennia. This is a moral or ethical position for physicians, not one of religion.
• Other than the ability to breathe air, there is no measurable difference between a newborn and what they were an hour before.
• With modern science, many babies that are born many months pre-mature grow up to be normal human beings (E.g 21 weeks). If they weren't real human beings when they were removed from the womb, when did they become human?
• Baby-like behaviour and responses occur long before birth.
• Even if it could be proven that a 12-hour old embryo isn't a human being while a 20-week old fetus is, there is no obvious place to draw the line; such decisions would have to be moral and arbitrary, and as such, could be wrong.
• If it is socially acceptable to terminate a fetus, then there is no scientific reason why it shouldn't be socially acceptable to terminate a newborn baby or a toddler.

But really, this is simply being defensive. With a defensive argument, one can either lose or tie, but never win.

The onus should be on the other side to defend their support for killing unborn children. But they really don't have any substantial argument beyond the lame "it's my body" or "it's my right"; their other arguments don't stand up:

• Rape or incest:
  Perhaps, but they constitute about onc abortion in a thousand.
  They don't in any way justify providing abortion-on-demand for the other 999 cases.

• Dangerous pregnancies:
  Again perhaps, and almost certainly if both will die without it, but again, this is less than a percent of all abortions.

• The fetus doesn't feel pain:
  Scientifically untrue. Pain responses can be seen as early as 12 weeks into the pregnancy.
  And even if it were true, it would be irrelevant. Adults can be poisoned and killed without pain, but that doesn't mean that it should be acceptable to do so.

• Being anti-abortion means being against women's rights:
  This is simply the false logic of conflating two issues. Many people support equal treatment and opportunity for men and women, quite independently of whether they think abortion-on-demand is acceptable.

• Abortion isn't “Killing”: Again, scientifically untrue. Fetuses are obviously alive by every definition of the word. After an abortion, they are dead. Clearly they were killed.

• Without access to abortion, many women would have to suffer for the rest of their lives:
  Yet again, scientifically untrue. Statistics show that:

  • If denied an abortion:
    • One week later, 65% still wish they could have had the abortion.
    • After the birth, 12% still wish they could have had it.
    • One year later, 7% still do.
    • By the child's 5^th birthday, only 4% still feel that way.
    • If the child was raised and not put up for adoption, that last figure is 2%.
    • Almost no one experiences long-term regret about not having an abortion.
  • For those that did successfully have an abortion, many do regret that decision for the rest of their lives.

— Summarized from Seven Lies About Abortion

It's somewhat ironic that the "pro-choice" people are so upset with the recent Supreme Court ruling that removed the previous restriction and granted the individual states the right to choose whether abortion should be allowed.

Let's consider 6 effective points of emphasis:

1. Humanize the human. If the preborn human being is recognized as a human person, rather than an inconsequential clump of cells, violence against that person becomes very difficult to justify. The most atrocious violations of human rights in history have been justified by dehumanizing the victim (source). I see an enormous red flag when proponents of abortion use an argument that was also used to enslave Africans & massacre Jews.

2. Show it for what it is. Although euphemisms and slogans may be catchy, most people who actually see what the procedure does are no longer able to ignore their conscience, and find it abhorrent.

3. Quantify the destruction. More human beings die globally by abortion every day than were killed in Hiroshima the day it was destroyed by the atomic bomb. More human beings have died globally by abortion in the last 3 decades than were killed by Mao, Stalin, Hitler, and Genghis Khan combined (sources for these truly ghastly statistics here, here, here, and here). As Alveda King poignantly observed, "abortion has done what the Klan only dreamed of. Roughly one-quarter of the Black population is missing." (source)

4. Recognize that human beings are created in the image of God (Genesis 1:27) and children are a gift from God (Psalm 127:3) formed and prepared by Him prior to birth (Jeremiah 1:5).

5. Give a voice to the women who have been deeply harmed physically and/or emotionally by abortion. The pro-choice movement seeks to hide these stories because they are inconvenient. Melinda Tankard Reist's Giving Sorrow Words - Women's Stories of Grief After Abortion painfully documents the terrible toll abortion takes on women, many of whom are coerced into getting an abortion and spend the rest of their lives regretting it. There is nothing remotely pro-woman about an abortion culture that does to women what Reist documents. The cruel irony is that many in the "pro-choice" camp do not want women to have any choice at all, except abortion (examples here & here).

6. Highlight that the principal beneficiaries of abortion on demand are sexually irresponsible men. Such men see abortion as a means to secure commitment-free sex (known to be psychologically harmful, especially to women), and as a license to abandon women they may impregnate. See further development of this point here and a withering satirical critique here.

Note that only one of the six arguments above is directly religious in nature. While many religious people are pro-life, so are many atheists & agnostics. The pro-life case requires no adherence to any specific religious doctrine, only a consistent belief in human rights. Christians avail themselves of both ethical and religious arguments in defending the right to life.

Post-script--response to comments

Begging the question

It was suggested that I commit the fallacy of begging the question in point #3. I have not done so. Note that many abortion proponents will deny the personhood of the unborn, but no serious arguments exist to suggest the the unborn human is not a human being.

The following working definitions may be helpful:

• Human being: a member of the species h*m* sapiens
• Alive: maintaining or working towards homeostasis

Point #3 does not address the personhood of the unborn (point #1 does); but regardless of the conclusions reached or not reached in point #1, point #3 about the death of human beings is still tragically true.

--

"Full-fledged human"

Some have sought to defend human rights on the basis of something other than a binary variable. The obvious binary choice is:

• If human_being => possesses_human_rights = TRUE
• Else possesses_human_rights = FALSE

Any attempt to defend human rights on the basis of a non-binary variable (skin color, IQ, size, level of development, ability to speak, degree of dependence, location etc.) falls easily to a reductio ad absurdum, as demonstrated by Seth Gruber here.

--

Violence

It is disappointing that the discussion in the comments ended with a post (since removed from the site) suggesting that physical violence against me was an appropriate/justified response to my pro-life arguments. If pro-abortion arguments themselves were clear & rational, their proponents would not need to resort to violent threats, mob action, and terrorism to achieve their goals. As the old debate adage says, "when the facts are on your side, pound the facts. When the facts are not on your side, pound the table."

This underscores the emphasis in the original question of the need to change hearts & minds on the subject of abortion. There is a need to transform from a culture of promiscuity, lust, violence, and death--to a culture of commitment, love, and life.

If you were just joining this world now, which would you want to live in?