6 After this, to indicate that we ought neither to feel confidence in prayer, while neglecting our own doings; nor, when taking pains, trust only to our own endeavors; but both to seek after the help from above, and contribute withal our own part; He sets forth the one in connection with the other. For so after much exhortation, He taught also how to pray, and when He had taught how to pray, He proceeded again to His exhortation concerning what we are to do; then from that again to the necessity of praying continually, saying, “Ask,” and “seek,” and “knock.” And thence again, to the necessity of being also diligent ourselves.
“For all things,” says He, “whatsoever ye would that men should do to you, do ye also to them.”
Summing up all in brief, and signifying, that virtue is compendious, and easy, and readily known of all men.
And He did not merely say, “All things whatsoever ye would,” but, “Therefore all things whatsoever ye would.” For this word, “therefore,” He did not add without purpose, but with a concealed meaning: “if you desire,” says He, “to be heard, together with what I have said, do these things also.” What then are these? “Whatsoever ye would that men should do to you.” Do you see how He has hereby also signified that together with prayer we need exact conversation? And He did not say, “whatsoever things you would to be done unto you of God, those do unto your neighbor;” lest you should say, “But how is it possible? He is God and I am man:” but, “whatsoever you would to be done unto you of your fellow servant, these things do thou also yourself show forth towards your neighbor.” What is less burdensome than this? What fairer?
Then the praise also, before the rewards, is exceeding great.
“For this is the law and the prophets.” Whence it is evident, that virtue is according to our nature; that we all, of ourselves, know our duties; and that it is not possible for us ever to find refuge in ignorance.
Source: Homilies on the Gospel of St. Matthew (New Advent)