Objection 1. It seems that not all dissimulation is a sin. For it is written (Luke 24:28) that our Lord "pretended [Douay: 'made as though'] he would go farther"; and Ambrose in his book on the Patriarchs (De Abraham i) says of Abraham that he "spoke craftily to his servants, when he said" (Genesis 22:5): "I and the boy will go with speed as far as yonder, and after we have worshipped, will return to you." Now to pretend and to speak craftily savor of dissimulation: and yet it is not to be said that there was sin in Christ or Abraham. Therefore not all dissimulation is a sin.
Objection 2. Further, no sin is profitable. But according to Jerome, in his commentary on Galatians 2:11, "When Peter [Vulgate: 'Cephas'] was come to Antioch:--The example of Jehu, king of Israel, who slew the priest of Baal, pretending that he desired to worship idols, should teach us that dissimulation is useful and sometimes to be employed"; and David "changed his countenance before" Achis, king of Geth (1 Samuel 21:13). Therefore not all dissimulation is a sin.
Objection 3. Further, good is contrary to evil. Therefore if it is evil to simulate good, it is good to simulate evil.
Objection 4. Further, it is written in condemnation of certain people (Isaiah 3:9): "They have proclaimed abroad their sin as Sodom, and they have not hid it." Now it pertains to dissimulation to hide one's sin. Therefore it is reprehensible sometimes not to simulate. But it is never reprehensible to avoid sin. Therefore dissimulation is not a sin.
On the contrary, A gloss on Isaiah 16:14, "In three years," etc., says: "Of the two evils it is less to sin openly than to simulate holiness." But to sin openly is always a sin. Therefore dissimulation is always a sin.
I answer that, As stated above (109, 3; 110, 1), it belongs to the virtue of truth to show oneself outwardly by outward signs to be such as one is. Now outward signs are not only words, but also deeds. Accordingly just as it is contrary to truth to signify by words something different from that which is in one's mind, so also is it contrary to truth to employ signs of deeds or things to signify the contrary of what is in oneself, and this is what is properly denoted by dissimulation. Consequently dissimulation is properly a lie told by the signs of outward deeds. Now it matters not whether one lie in word or in any other way, as stated above (110, 1, Objection 2). Wherefore, since every lie is a sin, as stated above (Question 110, Article 3), it follows that also all dissimulation is a sin.
Reply to Objection 1. As Augustine says (De QQ. Evang. ii), "To pretend is not always a lie: but only when the pretense has no signification, then it is a lie. When, however, our pretense refers to some signification, there is no lie, but a representation of the truth." And he cites figures of speech as an example, where a thing is "pretended," for we do not mean it to be taken literally but as a figure of something else that we wish to say. On this way our Lord "pretended He would go farther," because He acted as if wishing to go farther; in order to signify something figuratively either because He was far from their faith, according to Gregory (Hom. xxiii in Ev.); or, as Augustine says (De QQ. Evang. ii), because, "as He was about to go farther away from them by ascending into heaven, He was, so to speak, held back on earth by their hospitality."
Abraham also spoke figuratively. Wherefore Ambrose (De Abraham i) says that Abraham "foretold what he knew not": for he intended to return alone after sacrificing his son: but by his mouth the Lord expressed what He was about to do. It is evident therefore that neither dissembled.
Reply to Objection 2. Jerome employs the term "simulation" in a broad sense for any kind of pretense. David's change of countenance was a figurative pretense, as a gloss observes in commenting on the title of Psalm 33, "I will bless the Lord at all times." There is no need to excuse Jehu's dissimulation from sin or lie, because he was a wicked man, since he departed not from the idolatry of Jeroboam (2 Kings 10:29-31). And yet he is praised withal and received an earthly reward from God, not for his dissimulation, but for his zeal in destroying the worship of Baal.
Reply to Objection 3. Some say that no one may pretend to be wicked, because no one pretends to be wicked by doing good deeds, and if he do evil deeds, he is evil. But this argument proves nothing. Because a man might pretend to be evil, by doing what is not evil in itself but has some appearance of evil: and nevertheless this dissimulation is evil, both because it is a lie, and because it gives scandal; and although he is wicked on this account, yet his wickedness is not the wickedness he simulates. And because dissimulation is evil in itself, its sinfulness is not derived from the thing simulated, whether this be good or evil.
Reply to Objection 4. Just as a man lies when he signifies by word that which he is not, yet lies not when he refrains from saying what he is, for this is sometimes lawful; so also does a man dissemble, when by outward signs of deeds or things he signifies that which he is not, yet he dissembles not if he omits to signify what he is. Hence one may hide one's sin without being guilty of dissimulation. It is thus that we must understand the saying of Jerome on the words of Isaiah 3:9, that the "second remedy after shipwreck is to hide one's sin," lest, to wit, others be scandalized thereby.
Source: Summa Theologica (New Advent)