Thomas Aquinas

“ 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. ”

“ It seems that flattery is a mortal sin. For, according to Augustine (Enchiridion xii) , "a thing is evil because it is harmful." But flattery is most harmful, according to Ps. 9:24, "For the sinner is praised in the desires of his soul, and the unjust man is blessed. The sinner hath provoked the Lord." Wherefore Jerome says (Ep. ad Celant) : "Nothing so easily corrupts the human mind as flattery": and a gloss on Ps. 69:4, "Let them be presently turned away blushing for shame that say to me: 'Tis well, 'Tis well," says: "The tongue of the flatterer harms more than the sword of the persecutor." ”

“ 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 Ps. ”