Objection 1. It seems that hypocrisy is always a mortal sin. For Jerome says on Isaiah 16:14: "Of the two evils it is less to sin openly than to simulate holiness": and a gloss on Job 1:21 [St. Augustine on Psalm 63:7, "As it hath pleased the Lord," etc., says that "pretended justice is no justice, but a twofold sin": and again a gloss on Lamentations 4:6, "The iniquity... of my people is made greater than the sin of Sodom," says: "He deplores the sins of the soul that falls into hypocrisy, which is a greater iniquity than the sin of Sodom." Now the sins of Sodom are mortal sin. Therefore hypocrisy is always a mortal sin.
Objection 2. Further, Gregory says (Moral. xxxi, 8) that hypocrites sin out of malice. But this is most grievous, for it pertains to the sin against the Holy Ghost. Therefore a hypocrite always sins mortally.
Objection 3. Further, no one deserves the anger of God and exclusion from seeing God, save on account of mortal sin. Now the anger of God is deserved through hypocrisy according to Job 36:13, "Dissemblers and crafty men prove the wrath of God": and the hypocrite is excluded from seeing God, according to Job 13:16, "No hypocrite shall come before His presence." Therefore hypocrisy is always a mortal sin.
On the contrary, Hypocrisy is lying by deed since it is a kind of dissimulation. But it is not always a mortal sin to lie by deed. Neither therefore is all hypocrisy a mortal sin.
Further, the intention of a hypocrite is to appear to be good. But this is not contrary to charity. Therefore hypocrisy is not of itself a mortal sin.
Further, hypocrisy is born of vainglory, as Gregory says (Moral. xxxi, 17). But vainglory is not always a mortal sin. Neither therefore is hypocrisy.
I answer that, There are two things in hypocrisy, lack of holiness, and simulation thereof. Accordingly if by a hypocrite we mean a person whose intention is directed to both the above, one, namely, who cares not to be holy but only to appear so, in which sense Sacred Scripture is wont to use the term, it is evident that hypocrisy is a mortal sin: for no one is entirely deprived of holiness save through mortal sin. But if by a hypocrite we mean one who intends to simulate holiness, which he lacks through mortal sin, then, although he is in mortal sin, whereby he is deprived of holiness, yet, in his case, the dissimulation itself is not always a mortal sin, but sometimes a venial sin. This will depend on the end in view; for if this be contrary to the love of God or of his neighbor, it will be a mortal sin: for instance if he were to simulate holiness in order to disseminate false doctrine, or that he may obtain ecclesiastical preferment, though unworthy, or that he may obtain any temporal good in which he fixes his end. 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.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)