Objection 1. It seems that hypocrisy is not the same as dissimulation. For dissimulation consists in lying by deeds. But there may be hypocrisy in showing outwardly what one does inwardly, according to Matthew 6:2, "When thou dost an alms-deed sound not a trumpet before thee, as the hypocrites do." Therefore hypocrisy is not the same as dissimulation.
Objection 2. Further, Gregory says (Moral. xxxi, 7): "Some there are who wear the habit of holiness, yet are unable to attain the merit of perfection. We must by no means deem these to have joined the ranks of the hypocrites, since it is one thing to sin from weakness, and another to sin from malice." Now those who wear the habit of holiness, without attaining the merit of perfection, are dissemblers, since the outward habit signifies works of perfection. Therefore dissimulation is not the same as hypocrisy.
Objection 3. Further, hypocrisy consists in the mere intention. For our Lord says of hypocrites (Matthew 23:5) that "all their works they do for to be seen of men": and Gregory says (Moral. xxxi, 7) that "they never consider what it is that they do, but how by their every action they may please men." But dissimulation consists, not in the mere intention, but in the outward action: wherefore a gloss on Job 36:13, "Dissemblers and crafty men prove the wrath of God," says that "the dissembler simulates one thing and does another: he pretends chastity, and delights in lewdness, he makes a show of poverty and fills his purse." Therefore hypocrisy is not the same as dissimulation.
On the contrary, Isidore says (Etym. x): "'Hypocrite' is a Greek word corresponding to the Latin 'simulator,' for whereas he is evil within," he "shows himself outwardly as being good; hypo denoting falsehood, and krisis, judgment."
I answer that, As Isidore says (Etym. x), "the word hypocrite is derived from the appearance of those who come on to the stage with a disguised face, by changing the color of their complexion, so as to imitate the complexion of the person they simulate, at one time under the guise of a man, at another under the guise of a woman, so as to deceive the people in their acting." Hence Augustine says (De Serm. Dom. ii) that "just as hypocrites by simulating other persons act the parts of those they are not (since he that acts the part of Agamemnon is not that man himself but pretends to be), so too in the Church and in every department of human life, whoever wishes to seem what he is not is a hypocrite: for he pretends to be just without being so in reality."
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. Wherefore Gregory says (Moral.) that "hypocrites make God's interests subservient to worldly purposes, since by making a show of saintly conduct they seek, not to turn men to God, but to draw to themselves the applause of their approval:" and so they make a lying pretense of having a good intention, which they have not, although they do not pretend to do a good deed without doing it.
Reply to Objection 2. The habit of holiness, for instance the religious or the clerical habit, signifies a state whereby one is bound to perform works of perfection. 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. 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.
Source: Summa Theologica (New Advent)