136 So he who does the will of God is His friend and is honoured with this name. He who is of one mind with Him, he too is His friend. For there is unity of mind in friends, and no one is more hateful than the man that injures friendship. Hence in the traitor the Lord found this the worst point on which to condemn his treachery, namely, that he gave no sign of gratitude and had mingled the poison of malice at the table of friendship. So He says: “It was thou, a man of like mind, My guide and Mine acquaintance, who ever took pleasant meals with Me.” That is: it could not be endured, for you fell upon Him Who granted grace to you. “For if My enemy had reproached Me I could have borne it, and I would have hid Myself from him who hated Me.” An enemy can be avoided; a friend cannot, if he desires to lay a plot. Let us guard against him to whom we do not entrust our plans; we cannot guard against him to whom we have already entrusted them. And so to show up all the hatefulness of the sin He did not say: Thou, My servant, My apostle; but thou, a man of like mind with Me; that is: you are not My but your own betrayer, for you betrayed a man of like mind with yourself.
137. The Lord Himself, when He was displeased with the three princes who had not deferred to holy Job, wished to pardon them through their friend, so that the prayer of friendship might win remission of sins. Therefore Job asked and God pardoned. Friendship helped them whom arrogance had harmed.
138. These things I have left with you, my children, that you may guard them in your minds— you yourselves will prove whether they will be of any advantage. Meanwhile they offer you a large number of examples, for almost all the examples drawn from our forefathers, and also many a word of theirs, are included within these three books; so that, although the language may not be graceful, yet a succession of old-time examples set down in such small compass may offer much instruction.
Source: On the Duties of the Clergy (New Advent)