Objection 1. It seems that irony, which consists in belittling oneself, is not a sin. For no sin arises from one's being strengthened by God: and yet this leads one to belittle oneself, according to Proverbs 30:1-2: "The vision which the man spoke, with whom is God, and who being strengthened by God, abiding with him, said, I am the most foolish of men." Also it is written (Amos 7:14): "Amos answered... I am not a prophet." Therefore irony, whereby a man belittles himself in words, is not a sin.
Objection 2. Further, Gregory says in a letter to Augustine, bishop of the English (Regist. xii): "It is the mark of a well-disposed mind to acknowledge one's fault when one is not guilty." But all sin is inconsistent with a well-disposed mind. Therefore irony is not a sin.
Objection 3. Further, it is not a sin to shun pride. But "some belittle themselves in words, so as to avoid pride," according to the Philosopher (Ethic. iv, 7). Therefore irony is not a sin.
On the contrary, Augustine says (De Verb. Apost., Serm. xxix): "If thou liest on account of humility, if thou wert not a sinner before lying, thou hast become one by lying."
I answer that, To speak so as to belittle oneself may occur in two ways. First so as to safeguard truth, as when a man conceals the greater things in himself, but discovers and asserts lesser things of himself the presence of which in himself he perceives. To belittle oneself in this way does not belong to irony, nor is it a sin in respect of its genus, except through corruption of one of its circumstances. Secondly, a person belittles himself by forsaking the truth, for instance by ascribing to himself something mean the existence of which in himself he does not perceive, or by denying something great of himself, which nevertheless he perceives himself to possess: this pertains to irony, and is always a sin.
Reply to Objection 1. There is a twofold wisdom and a twofold folly. For there is a wisdom according to God, which has human or worldly folly annexed to it, according to 1 Corinthians 3:18, "If any man among you seem to be wise in this world, let him become a fool that he may be wise." But there is another wisdom that is worldly, which as the same text goes on to say, "is foolishness with God." Accordingly, he that is strengthened by God acknowledges himself to be most foolish in the estimation of men, because, to wit, he despises human things, which human wisdom seeks. Hence the text quoted continues, "and the wisdom of men is not with me," and farther on, "and I have known the science of the saints" [Vulgate: 'and I have not known the science of the saints'].
It may also be replied that "the wisdom of men" is that which is acquired by human reason, while the "wisdom of the saints" is that which is received by divine inspiration. Amos denied that he was a prophet by birth, since, to wit, he was not of the race of prophets: hence the text goes on, "nor am I the son of a prophet."
Reply to Objection 2. It belongs to a well-disposed mind that a man tend to perfect righteousness, and consequently deem himself guilty, not only if he fall short of common righteousness, which is truly a sin, but also if he fall short of perfect righteousness, which sometimes is not a sin. But he does not call sinful that which he does not acknowledge to be sinful: which would be a lie of irony.
Reply to Objection 3. A man should not commit one sin in order to avoid another: and so he ought not to lie in any way at all in order to avoid pride. Hence Augustine says (Tract. xliii in Joan.): "Shun not arrogance so as to forsake truth": and Gregory says (Moral. xxvi, 3) that "it is a reckless humility that entangles itself with lies."
Source: Summa Theologica (New Advent)