Thomas Aquinas

“ Accordingly we must reply that dissimulation or hypocrisy may be opposed to a virtue in two ways, in one way directly, in another way indirectly. Its direct opposition or contrariety is to be considered with regard to the very species of the act, and this species depends on that act's proper object. Wherefore since hypocrisy is a kind of dissimulation, whereby a man simulates a character which is not his, as stated in the preceding article, it follows that it is directly opposed to truth whereby a man shows himself in life and speech to be what he is, as stated in Ethic. ”

“ We must conclude, therefore, that hypocrisy is dissimulation, not, however, any form of dissimulation, but only when one person simulates another, as when a sinner simulates the person of a just man.
Reply to Objection 1: The outward deed is a natural sign of the intention. Accordingly when a man does good works pertaining by their genus to the service of God, and seeks by their means to please, not God but man, he simulates a right intention which he has not. ”

“ And so when a man puts on the habit of holiness, with the intention of entering the state of perfection, if he fail through weakness, he is not a dissembler or a hypocrite, because he is not bound to disclose his sin by laying aside the habit of holiness. ”

“ 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. ”

“ If, however, he were to put on the habit of holiness in order to make a show of righteousness, he would be a hypocrite and a dissembler.
Reply to Objection 3: In dissimulation, as in a lie, there are two things: one by way of sign, the other by way of thing signified. Accordingly the evil intention in hypocrisy is considered as a thing signified, which does not tally with the sign: and the outward words, or deeds, or any sensible objects are considered in every dissimulation and lie as a sign. ”

“ 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. ”

“ If, however, the end intended be not contrary to charity, it will be a venial sin, as for instance when a man takes pleasure in the pretense itself: of such a man it is said in Ethic. iv, 7 that "he would seem to be vain rather than evil"; for the same applies to simulation as to a lie.
It happens also sometimes that a man simulates the perfection of holiness which is not necessary for spiritual welfare. Simulation of this kind is neither a mortal sin always, nor is it always associated with mortal sin. ”

“ 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 ”

“ 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 Ps. 33, "I will bless the Lord at all times." ”