The Fifth Petition "is concerned with our poor, miserable conduct. Although we have the Word of God, believe in Him, obey Him, and submit to His will, and though His gifts and blessings nourish our lives, yet we do not live without sinning. Because we live in the world among people who sorely vex us and give us occasion for impatience, anger, revenge, and so on, we stumble every day and overstep our bounds. We also, as we have heard, have Satan coming up from behind to close in on every side and aim his attacks against all our earlier petitions. Amid such conflict, it is not possible always to stand firm.
"Here again, therefore, the need is great for us to pray and to call upon God, 'Dear Father, forgive us our trespasses.' Not that He does not forgive sins without our prayer or before we ask. In fact, before we prayed for it or ever thought about it, He gave us the Gospel, in which there is nothing but forgiveness. But here the point is that we should recognize and accept this forgiveness. It is the way of the flesh, in which we live our daily life, to distrust and disbelieve God and to stir itself up constantly with evil desires and devices, so that we sin every day in word and deed, doing what is wrong and omitting to do what is right. As a result, our conscience feels unrest, fears God's wrath and displeasure, and thus lets the comfort and assurance of the Gospel sink low. Therefore it is necessary to keep running to the Gospel and drawing comfort from it by means of this petition in order to receive our good conscience.
"This process, however, is to serve God's purpose, namely to break our pride and keep us in humility. For He has reserved the prerogative, in case anyone insists on his own goodness and despises others, to let him look into himself when this petition confronts him. He will find that he is no better than others and that in the presence of God everyone must duck his head and come into the joy of forgiveness only through the low door of humility. Let no one think that in this life he will ever reach the point where he does not need this forgiveness. In short, unless God keeps on forgiving us, we are lost.
"Thus this petition is really an appeal to God not to rivet His eyes on our sins nor to punish them as we daily deserve, but to deal with us according to His grace and forgive us as He promised, and so to give us a happy and cheerful conscience able to stand before Him in prayer. Where the heart is not right with God and cannot draw such confidence from His Gospel, it will never dare to pray. But such a confident and joyful heart can come from nowhere else than from the knowledge that our sins are forgiven.
"Meanwhile, a necessary yet comforting word is attached here: 'as we forgive those who trespass against us.' God has promised us the certain assurance that all is completely forgiven and pardoned, yet with the understanding that we are also to forgive our neighbor. For just as God in His grace forgives everything by which we sin much against Him every day, so we also must constantly forgive our neighbor who does us harm, violence, and injustic, treats us with abomindably shabby tricks, and the like. If you do not forgive, do not imagine that God will forgive you. But if you do forgive, you have the comfort and assurance that in heaven you are forgiven. But you are forgiven not on account of the forgiveness you granted to your neighbor, for God forgives completely and for nothing, out of pure grace and because He promised it, as the Gospel teaches. Rather, God has linked our forgiveness of our neighbor to God's forgiveness of us for our strengthening and assurance, and as a sign alongside the promise in Luke 6:37, which agrees with this petition, 'Forgive, and you will be forgiven.' Hence Christ repeats the promise immediately after the Lord's Prayer, Matthew 6:14, and says, 'For if you forgive men their trespasses, your heavenly Father also will forgive you,' etc." (Luther's Large Catechism, CPH 1978)