Why is acting not a sin?

Lying and deception are closely connected. According to St. Thomas Aquinas both are sinful, but a lie is more likely to be a mortal (or grievous) sin. In a way this is because a lie is a more stark deception than deception that does not involve a lie. For Aquinas and the Church they are both sins because they are both contrary to the virtue of truth (CCC 2484), and lying is more contrary to truth than deception is. Indeed lying is a sin contrary to the very act and purpose of speech (CCC 2485). Further, the gravity of individual lies and deceptions are measured against the degree to which they distort and injure truth.

The other reason traditionally given for why deception is less grievous than lying is the fact that deception involves more fault on the part of the one deceived than lying does, and thus they bear a greater share of responsibility for the resulting deception than they would if they were being bluntly lied to.

A more concise answer to your question is as follows: deceptive acting (or dissimulation) is usually a sin, though in certain cases it is only a minor sin.

Sources and References:

• Aquinas on Truth
• Aquinas on Lying
• Aquinas on Dissimulation and Hypocrisy
• Augustine, "On Lying"
• Augustine, "Against Lying"
• John Henry Newman, "Lying and Equivocation"
• John Henry Newman, Appendix 1.8 of Apologia Pro Vita Sua
• Catechism of the Catholic Church, 2464-2513

Is it sinful to deceive someone?

Answer: It may all depend one the circumstances of the moment.

St. Augustine wrote the first extensive treatise on lying (De Mendacio). In it he cites the case of a holy bishop, Firmus of Thagasta, who wished to protect a man who had sought refuge with him. The bishop was so careful of the truth that, rather than lying to the imperial officers who pursued the fugitive, he told them frankly that he would not reveal the man’s location. Firmus maintained this resolve even under torture, with the result that he was eventually brought before the emperor himself. But the emperor was so impressed with the bishop’s virtue that he both praised the bishop and pardoned the fugitive.

Augustine tells this story to provide a saintly witness for his argument that lying is always morally wrong, regardless of the circumstances, and to note that God is perfectly capable of extricating from trouble those who stand fast in the truth. His treatise has been widely cited ever since, and his viewpoint was endorsed by no less saintly a scholar than Thomas Aquinas. In the monumental Summa Theologiae, Thomas states the same position: “Therefore it is not lawful to tell a lie in order to deliver another from any danger whatever. Nevertheless it is lawful to hide the truth prudently, by keeping it back, as Augustine says” (II:110:3).

Can a Lie Ever Be Necessary?

Note that a solution to this conundrum could come in one of two forms. It may be that: (1) The immorality of lying admits of exceptions such that there is no objective evil, or at least no subjective evil (guilt), in lying to the thugs; or (2) a very careful definition of “lying” will show that speaking falsely to the thugs is not a lie at all. Great and holy thinkers have wrestled with both possibilities, but it is perhaps more logical to take up first the question of the definition of “lying.” By carefully defining our terms, will we find that there is a distinction between speaking falsely and lying, just as there is between killing and murder? Are some falsehoods not lies? What precisely does it mean to lie?

One of the stronger philosophical traditions, endorsed by Aquinas and discussed by Augustine, posits that lying is “deliberately speaking against one’s own mind.” (Throughout this discussion, “speaking” means any sort of communication.) This was the most common definition among the scholastics, and it became a staple of theological manuals by the first part of the 20th century. As Fr. John Hardon puts it in the Modern Catholic Dictionary, “When a person tells a lie, he or she deliberately says something that is contrary to what is on that person’s mind; there is a real opposition between what one says and what one thinks”.

The first thing to notice is that this definition emphasizes the moral intentionality of lying; the truth itself is not necessarily contradicted. If a person thinks something is true and deliberately states something to the contrary, he has incurred the moral guilt of lying. While this may be so subjectively, it leaves open the possibility that such a person, believing a falsehood, could actually speak the truth by speaking against his own mind.

Because this definition is divorced from the objective truth or falsity of the statement, many philosophers and theologians have sought an alternative definition. Some have proposed that the proper definition of “lying” is “speaking a falsehood with the intention of deceiving.” In the early 20th century, the article on “Lying” in the highly-regarded Catholic Encyclopedia dismissed this definition (also traceable to Augustine) as a new and minor opinion which raised more problems than it solved. By the late 20th century, however, it wyas precisely this definition that made it into the Catechism of the Catholic Church (see CCC 2482).

Refined Definitions and Exceptions

Some moralists have argued that we are obliged to state the strict truth no matter what the consequences, on the principle that the end does not justify the means. Unfortunately, this makes a presumption that most thinkers would not admit: that the only reason to shy away from the truth is fear of unpleasant consequences. In the case of the murderous thugs, however, most people believe they would be complicit in a grave evil if they were to reveal the location of the intended victim, and it is worth noting that they could be charged as accomplices under most legal systems. Other moralists, as we have seen, argue that we are not strictly obliged to speak the truth, but we must not speak falsely. We may, for example, try to change the subject, keep silence, or openly refuse to answer. But in many cases this also would be likely to betray the innocent, and even very moral onlookers might well ask—somewhat contemptuously—whether this was the best we could do.

To address this critical problem more effectively, a great many moralists have tried either to tweak the definition or to suggest grounds for exceptions. For example, some proponents of the first definition have argued that a person is not really speaking against his own mind if his conscience instructs him to say something false (for example, to save an innocent person). This is internally consistent, and we must certainly follow our conscience, but it also weakens the obvious meaning of “speaking against one’s mind” and, in any case, the explanation does not provide any principle by which properly to form the conscience. Therefore, its very subjectivity renders it morally unhelpful.

Regardless of definition, many others have suggested that the immorality of lying admits of exceptions. These argue, for example, that one is not obligated to tell the truth to an enemy, or that political leaders may speak falsely for reasons of state. Such exceptions may be permitted by the principle of double effect: Just as one can morally kill to defend someone’s life, so one can morally lie for a similar reason.

A Promising Lead?

I referred earlier to the difficulty of conceptualizing a perfect definition of lying that might reveal a solution to our dilemma. The magisterium of the Church has not endorsed any such definition, but it recently came very close to taking a small step in that direction. Over the past hundred years there has been a growing movement among moral theologians to tweak the definition of lying as follows: “To lie is to speak or act against the truth in order to lead into error someone who has the right to know the truth.” This very sentence, in fact, is taken from the initial edition of the Catechism of the Catholic Church (CCC 2483, 1994 edition).

When the Catechism was first published in French in 1994, and translated into other languages from the French, it contained the sentence quoted above, and so there was some speculation that the Holy See had finally decided to throw at least a modicum of magisterial weight behind this solution to our dilemma. This very precise definition, with its inclusion of the right to know, enables us to handle lying and falsehood in a manner very similar to the way we handle murder and killing. Through a person’s intention to use particular knowledge for an evil end, that person would presumably forfeit his right to know. Thus it would be morally acceptable to speak a falsehood to the murderous thugs. But we would no more call this “lying” than we would call an act of self-defense “murder.” - Is Lying Ever Right?

St. Augustine wrote that lies told in jest, or by someone who believes or opines the lie to be true are not, in fact, lies.

The Sacred Scriptures gives us some insights into the dilemma of lying:

The Old Testament and New Testament of the Bible both contain statements that God cannot lie and that lying is immoral (Num. 23:19,[54] Hab. 2:3,[55] Heb. 6:13–18).[56] Nevertheless, there are examples of God deliberately causing enemies to become disorientated and confused, in order to provide victory (2 Thess. 2:11;[57][58] 1 Kings 22:23;[59] Ezek. 14:9).

Various passages of the Bible feature exchanges that assert lying is immoral and wrong (Prov. 6:16–19; Ps. 5:6), (Lev. 19:11; Prov. 14:5; Prov. 30:6; Zeph. 3:13), (Isa. 28:15; Dan. 11:27), most famously, in the Ten Commandments: "Thou shalt not bear false witness" (Ex. 20:2–17; Deut. 5:6–21); Ex. 23:1; Matt. 19:18; Mark 10:19; Luke 18:20 a specific reference to perjury.

Other passages feature descriptive (not prescriptive) exchanges where lying was committed in extreme circumstances involving life and death. However, most Christian philosophers would argue that lying is never acceptable, but that even those who are righteous in God's eyes sin sometimes. Old Testament accounts of lying include:

• The midwives lied about their inability to kill the Israelite children. (Ex. 1:15–21).

• Rahab lied to the king of Jericho about hiding the Hebrew spies (Josh. 2:4–5) and was not killed with those who were disobedient because of her faith (Heb. 11:31).

• Abraham instructed his wife, Sarah, to mislead the Egyptians and say that she is his sister (Gen. 12:10). Abraham's story was strictly true – Sarah was his half sister – but intentionally misleading because it was designed to lead the Egyptians to believe that Sarah was not Abraham's wife for Abraham feared that they would kill him in order to take her, for she was very beautiful.

In the New Testament, Jesus refers to the Devil as the father of lies (John 8:44) and Paul commands Christians "Do not lie to one another" (Col. 3:9; cf. Lev. 19:11). In the Day of Judgement, unrepentant liars will be punished in the lake of fire. (Rev. 21:8; 21:27). - Lie (Wikipedia)

Please remember: You do not have to always tell the truth; you are simply obligated not to lie!

There are many reasons I might put on a cheerful appearance for someone with whom I am angry.

Some of these motivations are noble: Perhaps I recognize that the anger is because this other fellow has reminded me of one of my sins, or I am angry for a genuine wrong he has done me, but I recognize that my dealings with him be characterized by love, and so forth.

Some of these motivations are vile: I plan to kill him for the wrong he had done me, but I want him to be off his guard, and so on.

We can see that if the motivation for hiding our feelings is itself a good motivation, then its becomes harder to justify the condemnation.

It is also true that by adopting the cheerful demeanor, we are putting restraint on our anger, which will weaken its hold on us; this is also a good thing.

The chief danger is that we human beings are amazingly skilled at kidding ourselves; we routinely tell ourselves that we are doing things for some noble reason when we are simply indulging a vice of some sort.

So while it may not be a go-to-hell sin to put on the happy face when dealing with someone who has angered us, it is certainly wiser that we do not delay in letting our true feelings, and the reasons for them, be known.

It cannot be wrong to be discreet or to present a specific outward appearance to others for Jesus commands his disciples that when they fast they are to anoint themselves with oil and they are not to advertise the fact that they are fasting, Matthew 6:16.

Nor can it be wrong to deliberately adopt an appearance that is anonymous or that is unidentifiable for Jesus did so on the Emmaus road, maintaining that presence for quite some time until the two disciples realised to whom they had spoken, Luke 24:15.

But it is clearly wrong to deliberately put on an act in order to present oneself in a good light to others, deliberately enhancing oneself in order to give a false appearance since Jesus condemns such things as praying long prayers in public and deliberately displaying the physical effects of fasting, calling it 'hypocrisy', Matthew 5 and 6.

We are told to be honest one with another and to speak to anyone who has trespassed, privately, in order to avoid grudges, Matthew 18:15, not to pretend that nothing is wrong, if it is. There is no need to 'act' as if nothing was wrong, if indeed it is.