Therefore, I will say: "I know very well that I am an unworthy man, worthy to be the devil's brother (not Christ's or His saints'). But now Christ has said that I (as the one for whom He died and rose again, as well as for St. Peter, who was a sinner like me) am His brother, and in all sincerity He wants me to believe Him without any doubting and wavering. He does not want me to consider or pay attention to the fact that I am unworthy and full of sin."
He Himself does not consider or remember, as He justly could do; He has sufficient cause to take revenge and punish His disciples for what they have merited from Him. Rather, everything is forgotten and blotted out of the heart (Isa. 44:22; Acts 3:19)--indeed, put to death, covered, and buried.
Now He can say nothing about them except everything delightful and good. He greets them and speaks with them as kindly as with His faithful, dearest friends and good children, as if they had done nothing wrong and had not troubled the water, but had done everything good for Him, so that they have no worry or care in their hearts that He would remember it and reproach them or take revenge on them.
Because He does not want to know about that, but has it put to death and covered up, why would I not let that be true and thank, praise, and love my dear Lord from my heart that He is so gracious and merciful? Even though I am burdened with sins, I still should not go ahead and all these kind words which I hear about Him a lie and arrogantly cast away the brotherhood He has offered,. If I do not believe it, that is not good for me, but there is not for that reason anything false or lacking in Him.
If anyone now wants to burden himself with new sins and not let what He has forgotten be forgotten, He certainly will be sinning in a way which can no longer be forgotten or be helped. The Epistle to the Hebrews speaks about those who fell into sin by falling away from God's Word and calling it lies (Hebrews 6[:6]; 10 [:26]). This is called the sin against the Holy Spirit, which means holding the Son of God up to contempt, trampling Him underfoot, and profaning the Spirit of grace. May God preserve from this all who want to be Christians!
Unfortunately, there is too much of the former blindness and folly in which we have previously tarried, which ought now be gone and forgotten, since we have been made His brothers, if we will only accept it. If we cannot believe as strongly as we ought, then we should begin (like young children) to suck up at least a little spoonful of this milk (1 Peter 2:2) and not at all push it away from us until we become stronger.
(Luther's Works. American Edition. Church Postil III, pp. 30-31)