2 Corinthians 8:15
4 Let us then think with shame on the great benefits we have already received, the great benefits we are yet to receive. And if a poor man come to us and beg, let us receive him with much good will, comforting, raising him up with [our] words, that we ourselves also may meet with the like, both from God and from men. “For whatsoever ye would that they should do unto you, do ye also unto them.” Nothing burdensome, nothing offensive, does this law contain. 'What you would receive, that do,' it says.
The return is equal. And it said not, 'what you would not receive, that do not,' but what is more. For that indeed is an abstinence from evil things, but this is a doing of good things, in which the other is involved. Also He said not 'that do ye also wish, but do, to them.' And what is the advantage? “This is the Law and the Prophets.” Would you have mercy shown you? Then show mercy. Would you obtain forgiveness? Then grant it. Would you not be evil spoken of? Then speak not evil.
Longest thou to receive praise? Then bestow it. Would you not be wronged? Then do not thou plunder. Do you see how He shows that virtue is natural, and that we need no external laws nor teachers? For in the things we wish to receive, or not to receive from our neighbors, we legislate unto ourselves. So that if you would not receive a thing, yet doest it, or if you would receive it, yet doest it not, you have become self-condemned and art henceforth without any excuse, on the ground of ignorance and of not knowing what ought to be done.
Wherefore, I beseech you, having set up this law in ourselves for ourselves, and reading this that is written so clearly and succinctly, let us become such to our neighbors, as we would have them be to ourselves; that may we both enjoy present immunity, and obtain the future good things, though the grace and love towards men of our Lord Jesus Christ, with Whom to the Father, together with the Holy Spirit, be glory, power, honor, now and for ever, and world without end. Amen.
Source: Homilies on Second Corinthians (New Advent)