Melancthon wrote to a guy called Brenz to clarify the difference between the Protestant position on justification and Augustine’s. The difference is vital!
Luther being Luther, he couldn’t help adding a P.S. to Melancthon’s letter:
And I, dear Brenz, in order to get a better grip on this issue frequently imagine it this way: as if in my heart there is no quality that is called faith or charity, but instead of them I put Christ himself and say: this is my righteousness; He is the quality and my formal righteousness, as they call it. In this way I free myself from the perception of the law and works, and even from the perception of this object, Christ, who is understood as a teacher or a giver; but I want Him to be my gift and teaching in Himself, so that I may have all things in Him. So he says: I am the way, the truth and the life. He does not say: I give you the way, the truth and the life, as if He worked in me while being placed outside of me. He must be such things in me, remain in me, live in me, speak not through me but into me, 2 Cor. 5; so that we may be righteousness in Him, not in love or in gifts that follow.