If a sin can be dispensed, is it really a sin?

Preface

I will try to post an answer to my own question since so many seem to be perturbed by the question.

Framing an Answer

I would say that with a precept of the Church such as the obligation to attend Sunday Mass we have a convergence of eternal law and positive ecclesiastical law. Indeed the Church is funneling different aspects of the eternal law into a concrete precept. Transgressing the obligation is a mortal sin precisely because one is transgressing the eternal law that the precept focuses into a concrete act. The Church does not have the power to simply create mortal sins on her own power or authority. Mortal sin is an irreparable defect consisting in the corruption of the principle of charity within us (ST Ia IIae, Q. 88, A. 1). It is by definition a sin against God that can only be healed by God. The Church's role with regard to mortal sin is only mediatory. She can recognize which sins are mortal and absolve them through God's power, but She cannot mitigate or dispense an act with intrinsically grave matter (such as murder).

There are at least five aspects of the eternal law that seem to come into play in the precept to attend Sunday Mass:

1. The third commandment: keep holy the Sabbath
2. Christ's command to "do this in remembrance of me" (Luke 22:19, 1 Corinthians 11:24-25)
3. Christ's resurrection appearances on "the eighth day" when the disciples were gathered together
4. The encouraging of communal worship, "For where two or three are gathered in my name, there am I in the midst of them" (Matthew 18:20)
5. Man's general obligation to worship God (ST IIae IIae, Q. 81)

The Church sees Sunday obligation as a combination of all of these different factors, along with the richness of the liturgy which has been passed down over the ages. Missing Mass is not a mortal sin because one has disobeyed an earthly authority, but rather because one has transgressed the eternal law as distilled in this precept.

Answering Specific Questions

1. If a sin can be dispensed, is it really a sin? Or more precisely: If an act which is usually considered a mortal sin can become non-sinful with a dispensation, is it really a mortal sin?

In the case of a dispensation from the Sunday obligation one is being dispensed from a Church precept, not the eternal law that undergirds the precept. Eternal law is always binding, and the Church has no more power to dispense from the eternal law than She has power to dispense from the prohibition against murder. For example, dispensing one from the obligation to attend Mass does not dispense one from the obligation to worship God, as the latter is not dispensable. Even during those times in our lives when we cannot attend Mass we are still obligated to worship God. The Sunday obligation is a concrete instantiation of eternal law and it is precisely that concrete instantiation that one is dispensed of.

Nevertheless, transgressing the Sunday obligation is a mortal sin due to the fact the obligation is an instantiation of eternal law. One is not merely transgressing Church authority, but is transgressing God's eternal law that underlies the precept.

2. Does the Church have the power to declare that something is now a sin which was not previously a sin, and to declare that something is no longer a sin which was previously a sin?

The Church can make these declarations on Her own power when it comes to venial sin, but not mortal sin. The Church cannot declare or stipulate mortal sin, she can only mediate the eternal law in making a judgment about mortal sin. In this case she mediates eternal law with the particular obligation to attend Mass. The eternal law that She mediates is something She receives from God; the positive law that she promulgates is something that comes from Her. Again, it is a combination or a mixture.

3. Why are the sins of murder and missing Mass treated so differently? What is the fundamental difference between the two? Do they have the same degree of severity?

The prohibition against murder comes directly from God without the mediation of the Church. It is part of the eternal law (as well as the natural law), "Thou shalt not kill" (Exodus 20:13). The prohibition against murder is not a combination or mixture of eternal law and positive ecclesiastical law, whereas the prohibition against missing Mass is.

If a sin can be dispensed, is it really a sin?

Let us first of all recall what Our Lord said to St. Peter, our first pope and head of the Catholic Church:

19 And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven. - Matthew 16:19

The Church dispenses of the obligation of a Church Law, not the sin because in reality of the act itself is not a sin due to such a dispensation. The title is playing with words.

The title of this question is poorly phrased to say the least!

Andreas Blass has this to say in the comments:

Let me try to emphasize part of what's wrong with the title: Sins cannot be dispensed; people can be dispensed from obligations, including ones that would ordinarily bind under pain of sin. The Church can dispense from its own laws, but not from natural or divine law. Examples of the latter are the law forbidding murder and the law requiring us to worship God. An example of the former is the Church's more specific requirement that our worship of God must include Sunday Mass.

There is no sin that exists when missing Mass through a dispensation from properly instituted authority (local ordinary).

Due to the present climate of this due to the coronavirus-19, the Catholic Church has dispensed, almost universally, of the obligation to attend Sunday Mass.

In the Catholic Church, the Precepts of the Church, sometimes called Commandments of the Church, are certain laws considered binding on the faithful. As usually understood, they are moral and ecclesiastical, broad in character and limited in number. In modern times there are five. These specifically Catholic commandments are additional to the Ten Commandments which are common to all the Abrahamic religions.

The Catechism of the Catholic Church enumerates the following five:

• 1. You shall attend Mass on Sundays and holy days of obligation.
• 2. You shall confess your sins at least once a year.
• 3. You shall humbly receive your Creator in Holy Communion at least during the Easter season.
• 4. You shall observe the prescribed days of fasting and abstinence.
• 5. You shall help to provide for the needs of the Church.

The Church does not dispense of the sin for not going to Sunday Mass because the Sunday obligation itself is dispensed of and the “sin” for abstaining from Sunday Mass no longer exists, due to very serious circumstances that exist.

2181 The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin.

If fact the idea of a Sunday Obligation did not come into Church law until the 4th century and afterwards:

The obligation of rest from work on Sunday remained somewhat indefinite for several centuries. A Council of Laodicea, held toward the end of the fourth century, was content to prescribe that on the Lord's Day the faithful were to abstain from work as far as possible. At the beginning of the sixth century St. Caesarius, as we have seen, and others showed an inclination to apply the law of the Jewish Sabbath to the observance of the Christian Sunday. The Council held at Orléans in 538 reprobated this tendency as Jewish and non-Christian. From the eight century the law began to be formulated as it exists at the present day, and the local councils forbade servile work, public buying and selling, pleading in the law courts, and the public and solemn taking of oaths. There is a large body of civil legislation on the Sunday rest side by side with the ecclesiastical. It begins with an Edict of Constantine, the first Christian emperor, who forbade judges to sit and townspeople to work on Sunday. He made an exception in favour of agriculture. The breaking of the law of Sunday rest was punished by the Anglo-Saxon legislation in England like other crimes and misdemeanours. After the Reformation, under Puritan influence, many laws were passed in England whose effect is still visible in the stringency of the English Sabbath. Still more is this the case in Scotland. There is no federal legislation in the United States on the observance of the Sunday, but nearly all the states of the Union have statutes tending to repress unnecessary labour and to restrain the liquor traffic. In other respects the legislation of the different states on this matter exhibits considerable variety. On the continent of Europe in recent years there have been several laws passed in direction of enforcing the observance of Sunday rest for the benefit of workmen. - Sunday (Catholic Encyclopaedia)

Only those who ”deliberately” fail in the obligation to attend Sunday Mass without a dispensation commit a grave sin.

This condition still exists, but we are dispensed now of the above mentioned obligation. People are dispensed, not sin!

In the present worldwide situation the Catholic Church finds herself in, she has deemed it prudent to dispense the faithful of attending the Eucharistic celebration on Sundays.

It sucks, but that is the way it goes!

I am trying to supplement Ken Graham's excellent answer because zippy2006 says it has not addressed the core issue posed in the question. My purpose is more of giving perspective, not trying to be canon lawyer, theologian, liturgist, or procedural expert.

"If an obligation that would ordinarily bind under pain of mortal sin can be dispensed with, then is it really an obligation that ought to bind under pain of mortal sin?" I don't think it takes much effort at all to understand what the title means. Indeed, many are fixated on the technical sense of "dispense" without taking into account the organic meaning of the word. The title expresses the essence of the question and gets at the central issue of whether what is being prohibited is gravely sinful. Whether dispensation can apply to sin-prohibitions is precisely the point of the question.

I think we need to consider why the obligation exists in the first place, before talking about dispensation. Why does the Catholic Church consider it a grave matter to attend weekly Sunday Mass in the first place? (Note: see also my answer to a follow up question on more of the history of the obligation.)

The March 1997 article "Attendance at Mass and participation in the liturgy" from The Angelus magazine explains in a great detail the essence of attending mass as well as helping us to understand the essence by attending to various circumstances, some of which give light to why the Catholic Church currently dispenses her members from attending Sunday Mass due to Corona Virus. Some quotes from the article will follow the answer.

Answer

The purpose of attending weekly Sunday Mass is "to prove to God that [the faithful] really loves Him and wants to receive all the graces necessary for his eternal salvation." To miss these graces is literally a matter of spiritual life and death. That is why the Catholic Church consider it grave matter, and appropriately assigned "mortal sin" for violating it. But can the Church force us to love God? This is between us and God. So what can the Church do?

I think the Jewish practice of coming up with the extra prohibition/obligation as a "fence" to protect Jews from violating the 613 commandments can be used here as an analogy. We humans have to battle laziness, distraction, concupiscence, etc. and in Her wisdom the Catholic Church knew that we would rather stay home and do other things than going to mass. So in aiming at leading all faithful to Christ, the Church decided to promulgate this obligation as a "fence" herding unruly sheep to the right path so we meet God properly every Sunday so we have better chance to be in the state of grace.

By dispensing the Sunday obligation, the Church is in no way diminishing the seriousness of the purpose of the Sunday Mass in the first place, which is "to prove to God that [the faithful] really loves Him and wants to receive all the graces necessary for his eternal salvation." It is just that the mode of herding has to be temporarily adjusted in light of the circumstances. Just like murder, not loving God is still a mortal sin. Nothing changes here. Now that the faithful cannot go Sunday mass, the faithful still has the obligation to show God our love and still has to be replaced in a similar manner as people who qualify for one of the 3 exemptions that have been in place before the Corona virus dispensation.

Therefore, as Ken Graham has explained through another reasoning, the essence of the grave matter causing the mortal sin is unchanged, making the example of Corona virus irrelevant for your question.

Quotes from the article (emphasis mine)

The Church aims at leading all men to Christ, Who gives all honor and glory to God the Father in union with the Holy Ghost. This is, in fact, the reason for our existence: to glorify God, i.e., by recognizing His transcendence, His majesty, His power, His goodness and by singing His praises.

And no human being can do this except through Christ, with Him and in Him. This is likewise the first purpose of the Holy Sacrifice of the Mass.

That is why everyone should go to Mass: not just because they must or to avoid committing a mortal sin, but in order to participate in the great liturgy of our Mother the Church, Who wants to gather Her children for this great “action” of Her Spouse. This “action” proclaims to God the Father that His children love Him as well as His beloved Son.

If we would better understand why there is a Church precept to go to Mass, let us recall to mind here the fourfold purpose of the sacrifice of the Cross, which is the same purpose as in the Holy Sacrifice of the Mass.

• The first is adoration: to acknowledge the greatness, majesty and power of God, and to humble oneself before Him.
• The second is thanksgiving: to thank Him for the many gifts, both universal and personal, for many graces, for so much goodness and mercy towards us, poor sinners.
• The third is propitiation, to make reparation and expiation for our faults, and to implore pardon of God for all sins, whether known or unknown, whether committed by ourselves or by others.
• The fourth is petition: to expose to God all our needs and to pray to Him for those who need His graces.

No human being can afford to neglect the accomplishment of these four duties, if he wants to prove to God that he really loves Him and wants to receive all the graces necessary for his eternal salvation. However nobody except our Lord Jesus Christ can wholly fulfill these duties, Whose homage is pleasing and acceptable to God, because of the purity of His love and the offering of Himself. Our Lord Jesus Christ comes to us with the holy and pure gift of Himself, through the hands of the Church and her ministers. He invites us to present our gifts, miserable and imperfect though they be, to combine them with His own so that they may be acceptable to God the Father.

Thus, Sunday Mass summarizes all our prayers from the previous week, and all our efforts and meritorious acts, even those of the following week are contained in the holy oblation of our Lord in such a way that they are rendered pleasing and acceptable to His Father. Isn’t this an outstanding proof of goodness and tenderness? Do we really need to be forced by the Church to go and have recourse to our Lord, to tell Him that we love Him, to unite ourselves to His sacrifice in order to receive His Divinity in return, and His soul, His Body, His Blood, His life given up for us, His patience and power, strength and goodness together with the immense blessing which the Father has reserved for his Son and for all those who resemble Him?

Our Sunday Obligation

Let us remark first of all that this precept is not restrictive. True, it is both allowed and even encouraged to spend an important part of Sunday in religious activities in order to better sanctify Sunday, e.g., to assist at a longer Mass rather than a short one, to attend Vespers or Compline; to spend more time with the family in relaxation, music, walks, games, prayers, good readings or visits to a shrine....

But the Church obliges, strictly speaking, only attendance at Mass. Moreover, this is an obligation for each and every Catholic aged seven and older, under pain of mortal sin, and there are certain conditions to be fulfilled which follow. We will not speak here about dress code, although this is likewise an important issue.

...

Although the Church cannot exempt us from keeping Sundays holy, since this is a divine precept, She can dispense us from doing it by assisting at Mass, an ecclesiastical precept.

There are three causes which may exempt from the Sunday obligation:

1. Necessity: physical or moral impossibility such as sickness, convalescence, nervous problems because of the crowd, too far to drive, etc.

2. Duties of state: soldiers, doctors, nurses, firemen on duty, mothers with very young children, etc.

3. Charity: the needs of our neighbor, such as taking care of the sick, etc.

In all these cases it is not required to make up by attending Mass during the week, even though that would be laudable; but it is still necessary to sanctify Sunday in some other way as best one can. The best thing would be to spend as much time privately in prayer, as one would have otherwise spent at Mass on Sunday.

The above remarks apply only to Masses which are an obligation. There is no sin whatever in missing parts of a weekday Mass.

...

"If a sin can be dispensed, is it really a sin?"

No, a sin which is dispensed of could never be a sin, because a sin is an offense to God, and offending God can never be excused.

"Does the Church have the power to declare that something is now a sin which was not previously a sin, and to declare that something is no longer a sin which was previously a sin?"

No, the Church cannot dispense someone from their moral duty as creatures of God to avoid sinful activity.

However, the Church can remove the obligation to do something in a certain way (i.e. attend Mass as a species of activity which falls under the genus 'keeping holy or separate the Sabbath day'). By removing the obligation, they remove the possibility of contravening any law, because it is no longer binding.

In this case, the Church has not modified what is intrinsically sinful (excusing people from offending God), but have modified what Christians are bound by obedience to the Church to do.

To use @Geremia's example: if a parent tells their child not to eat a cookie, it is wrong to for that child to eat the cookie. If a parent then permits the child to eat a cookie, then it is not wrong for the child to eat the cookie. In this instance, eating cookies, or not eating cookies is utterly irrelevant - only the obedience to one's parents.

Similarly, every Christian has the moral duty to obey the commandments of the Church as their legitimate authority and teacher - and per the perennial trope, mother. Attending or not attending Mass is not intrinsically or inseparably related to keeping holy the Sabbath day, as is proven by the sheer fact that the Mass did not exist until Calvary - one thousand years after the commandment was issued by God.

Attending Mass is one of many ways to keep holy the Sabbath day, and thus is not necessarily commanded by the commandment to keep holy the Sabbath day (i.e. where Christians have always viewed Sunday as the new Sabbath of Christians).

Thus, the Church, in her prudence, or even simply by her authority, has the right to bind or loose the obligation to attend Mass on Sunday, without touching the sinfulness of obedience, which remains, whether one is bound by obedience to attend Mass on Sunday, or freed from the obligation by the Church. To date, the Church has not seen any good reason to dispense anyone from attending Mass every Sunday except for relatively extreme travel requirements, illness, or, apparently, in times of the current crisis.

"Why are the sins of murder and missing Mass treated so differently? What is the fundamental difference between the two? Do they have the same degree of severity?"

A mortal sin is simply any sin which leads to spiritual death (hence "mortal," which means "causing death"). This doesn't imply that all sins which are mortal have the same severity, incur the same punishment in hell (or temporal punishment, if forgiven). A child abuser will incur more punishment for his sin, than someone who elects not to travel to attend Mass because he has a bad case of the sniffles. For reasons that should be obvious. Murder is obviously worse than wilfully lying about a non-trivial matter (such as making a false oath). Homosexual fornication is obviously worse than heterosexual fornication, because it involves at least one more degree of deviation from God-ordained sexual relations than does heterosexual. Etc.

That is, while venial and mortal sins designate two manners of sinning, they don't imply that all venial, or all mortal, sins are the same as every other, or carry the same penalty.