For He sets before every race of mankind that which is always and universally just, as well as all righteousness; and every race knows that adultery, and fornication, and homicide, and such like, are sinful; and though they all commit such practices, yet they do not escape from the knowledge that they act unrighteously whenever they so do, with the exception of those who are possessed with an unclean spirit, and who have been debased by education, by wicked customs, and by sinful institutions, and who have lost, or rather quenched and put under, their natural ideas. For we may see that such persons are unwilling to submit to the same things which they inflict upon others, and reproach each other with hostile consciences for the acts which they perpetrate. And hence I think that our Lord and Saviour Jesus Christ spoke well when He summed up all righteousness and piety in two commandments. They are these: 'You shall love the Lord your God with all your heart, and with all your strength, and your neighbour as yourself.' For the man who loves God with all the heart, and with all the strength, being filled with a God-fearing mind, will reverence no other god; and since God wishes it, he would reverence that angel who is beloved by the same Lord and God. And the man who loves his neighbour as himself will wish for him the same good things that he wishes for himself, and no man will wish evil things for himself. Accordingly, he who loves his neighbour would pray and labour that his neighbour may be possessed of the same benefits as himself. Now nothing else is neighbour to man than that similarly-affectioned and reasonable being—man. Therefore, since all righteousness is divided into two branches, namely, in so far as it regards God and men, whoever, says the Scripture, loves the Lord God with all the heart, and all the strength, and his neighbour as himself, would be truly a righteous man.
Justin, Dialogue with Trypho, Chapter 93. St. Justin, of course, was a Middle Platonist philosopher who was martyred in the persecutions under Marcus Aurelius, a philosopher, possibly at the instigation of Crescens, a philosopher. I talked a bit about his philosophical context last year. He's most famous, perhaps, for his view that Socrates and other righteous pagans were proto-Christians, for they devoted themselves to Logos (=Word, Reason) and, as the Gospel of John tells us, the Logos is Christ.
