Objection 1. It seems that flattery is not a sin. For flattery consists in words of praise offered to another in order to please him. But it is not a sin to praise a person, according to Proverbs 31:28, "Her children rose up and called her blessed: her husband, and he praised her." Moreover, there is no evil in wishing to please others, according to 1 Corinthians 10:33, "I... in all things please all men." Therefore flattery is not a sin.
Objection 2. Further, evil is contrary to good, and blame to praise. But it is not a sin to blame evil. Neither, then, is it a sin to praise good, which seems to belong to flattery. Therefore flattery is not a sin.
Objection 3. Further, detraction is contrary to flattery. Wherefore Gregory says (Moral. xxii, 5) that detraction is a remedy against flattery. "It must be observed," says he, "that by the wonderful moderation of our Ruler, we are often allowed to be rent by detractions but are uplifted by immoderate praise, so that whom the voice of the flatterer upraises, the tongue of the detractor may humble." But detraction is an evil, as stated above (73, 2,3). Therefore flattery is a good.
On the contrary, A gloss on Ezekiel 13:18, "Woe to them that sew cushions under every elbow," says, "that is to say, sweet flattery." Therefore flattery is a sin.
I answer that, As stated above (114, 1, ad 3), although the friendship of which we have been speaking, or affability, intends chiefly the pleasure of those among whom one lives, yet it does not fear to displease when it is a question of obtaining a certain good, or of avoiding a certain evil. Accordingly, if a man were to wish always to speak pleasantly to others, he would exceed the mode of pleasing, and would therefore sin by excess. If he do this with the mere intention of pleasing he is said to be "complaisant," according to the Philosopher (Ethic. iv, 6): whereas if he do it with the intention of making some gain out of it, he is called a "flatterer" or "adulator." As a rule, however, the term "flattery" is wont to be applied to all who wish to exceed the mode of virtue in pleasing others by words or deeds in their ordinary behavior towards their fellows.
Reply to Objection 1. One may praise a person both well and ill, according as one observes or omits the due circumstances. For if while observing other due circumstances one were to wish to please a person by praising him, in order thereby to console him, or that he may strive to make progress in good, this will belong to the aforesaid virtue of friendship. But it would belong to flattery, if one wished to praise a person for things in which he ought not to be praised; since perhaps they are evil, according to Psalm 9:24, "The sinner is praised in the desires of his soul"; or they may be uncertain, according to Sirach 27:8, "Praise not a man before he speaketh," and again (Sirach 11:2), "Praise not a man for his beauty"; or because there may be fear lest human praise should incite him to vainglory, wherefore it is written, (Sirach 11:30), "Praise not any man before death." Again, in like manner it is right to wish to please a man in order to foster charity, so that he may make spiritual progress therein. But it would be sinful to wish to please men for the sake of vainglory or gain, or to please them in something evil, according to Psalm 52:6, "God hath scattered the bones of them that please men," and according to the words of the Apostle (Galatians 1:10), "If I yet pleased men, I should not be the servant of Christ."
Reply to Objection 2. Even to blame evil is sinful, if due circumstances be not observed; and so too is it to praise good.
Reply to Objection 3. Nothing hinders two vices being contrary to one another. Wherefore even as detraction is evil, so is flattery, which is contrary thereto as regards what is said, but not directly as regards the end. Because flattery seeks to please the person flattered, whereas the detractor seeks not the displeasure of the person defamed, since at times he defames him in secret, but seeks rather his defamation.
Source: Summa Theologica (New Advent)