Lactantius on the Golden Rule

"When the affairs of men were in this condition, God pitied us, revealed and displayed Himself to us, that in Himself we might learn religion, faith, purity, and mercy; that having laid aside the error of our former life, together with God Himself we might know ourselves, whom impiety had disunited from Him, and we might choose the divine law, which unites human affairs with heavenly, the Lord Himself delivering it to us; by which law all the errors with which we have been ensnared, together with vain and impious superstitions, might be taken away. What we owe to man, therefore, is prescribed by that same divine law which teaches that whatever you render to man is rendered to God. But the root of justice, and the entire foundation of equity, is that you should not do that which you would be unwilling to suffer, but should measure the feelings of another by your own. If it is an unpleasant thing to bear an injury, and he who has done it appears unjust, transfer to the person of another that which you feel respecting yourself, and to your own person that which you judge respecting another, and you will understand that you act as unjustly if you injure another as another would if he should injure you. If we consider these things, we shall maintain innocence, in which the first step of justice is, as it were, contained. For the first thing is, not to injure; the next is, to be of service. And as in uncultivated lands, before you begin to sow, the fields must be cleansed by tearing up the thorns and cutting off all the roots of trunks, so vices must first be thrust out from our souls, and then at length virtues must be implanted, from which the fruits of immortality, being engendered by the word of God, may spring up." (Lactantius, Epitome of the Divine Institutes 60)