18 “Let your light,” says He, “so shine before men, that they may see your good works, and glorify your Father which is in heaven.” If He had merely said, “Let your light so shine before men, that they may see your good works,” He would seem to have fixed an end in the praises of men, which hypocrites seek, and those who canvass for honours and covet glory of the emptiest kind. Against such parties it is said, “If I yet pleased men, I should not be the servant of Christ;” and, by the prophet, “They who please men are put to shame, because God has despised them;” and again, “God has broken the bones of those who please men;” and again the apostle, “Let us not be desirous of vainglory;” and still another time, “But let every man prove his own work, and then shall he have rejoicing in himself alone, and not in another.” Hence our Lord has not said merely, “that they may see your good works,” but has added, “and glorify your Father who is in heaven:” so that the mere fact that a man by means of good works pleases men, does not there set it up as an end that he should please men; but let him subordinate this to the praise of God, and for this reason please men, that God may be glorified in him. For this is expedient for them who offer praise, that they should honour, not man, but God; as our Lord showed in the case of the man who was carried, where, on the paralytic being healed, the multitude, marvelling at His powers, as it is written in the Gospel, “feared and glorified God, which had given such power unto men.” And His imitator, the Apostle Paul, says, “But they had heard only, that he which persecuted us in times past now preaches the faith which once he destroyed; and they glorified God in me.”
19. And therefore, after He has exhorted His hearers that they should prepare themselves to bear all things for truth and righteousness, and that they should not hide the good which they were about to receive, but should learn with such benevolence as to teach others, aiming in their good works not at their own praise, but at the glory of God, He begins now to inform and to teach them what they are to teach; as if they were asking Him, saying: Lo, we are willing both to bear all things for Your name, and not to hide Your doctrine; but what precisely is this which Thou forbiddest us to hide, and for which You command us to bear all things? Are You about to mention other things contrary to those which are written in the law? “No,” says He; “for think not that I have come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.”
Source: Our Lord's Sermon on the Mount (New Advent)