[The penance] which is required of us and which brings us back to favor with the Lord, must never, once we have known and embraced it, be violated thereafter by a return to sin. In this case, no plea of ignorance excuses you…
The man who began to satisfy the Lord by repenting his sin will satisfy the devil by repenting his repentance, and he will be as hateful to the Lord as he is dear to his adversary.
Some say, however, that God is satisfied if He be honored in heart and mind, even though this be not done externally. Thus they sin, yet lose not reverential fear and faith. That is to say, they lose not chastity and commit adultery! They lose not filial piety and poison a parent!… A wonderful example of wrong-headedness.
(Tertullian, On Penitence, ACW 28, p. 22, 23 in Oden, T. C. 1989. Ministry Through Word and Sacrament (150–151). Crossroad: New York)