St Cyprian's treatise on the Lord's Prayer

After the support of bread, we ask for the forgiveness of sins

As the prayer continues, we ask Give us this day our daily bread. This can be understood both spiritually and literally, because either way of understanding is rich in divine usefulness to our salvation. For Christ is the bread of life, and this bread does not belong to anyone at all, but to us. And so, just as we say Our Father, because he is the father of those who understand and believe, so also we call it our bread, because Christ is the bread of us who come into contact with his body.
We ask that this bread should be given to us daily, that we who are in Christ and daily receive the Eucharist as the food of salvation may not be prevented, by the interposition of some heinous sin, from partaking of the heavenly bread and be separated from Christ’s body, for as he says: I am the bread of life which came down from heaven. If anyone eats of my bread, he will live for ever; and the bread I shall give is my flesh, for the life of the world.
So when he says that whoever eats of his bread will live for ever; and as it is clear that those are indeed living who partake of his body and, having the right of communion, receive the Eucharist, so, on the other hand, we must fear and pray lest anyone should be kept at a distance from salvation who, being withheld from communion, remains separate from Christ’s body. For he has given us this warning: Unless you eat the flesh of the Son of man and drink his blood, you will have no life in you. And therefore we ask that our bread – that is, Christ – may be given to us daily, so that we who live in Christ may not depart from his sanctification and his body.
After this we entreat for our sins, saying Forgive us our debts, as we forgive our debtors. After the supply of food, pardon of sin is also asked for.
How necessary, how provident, how salutary are we reminded that we are sinners, since we have to beg for forgiveness, and while we ask for God’s pardon, we are reminded of our own consciousness of guilt! Just in case anyone should think himself innocent and, by thus exalting himself, should more utterly perish, he is taught and instructed that he sins every day, since he is commanded to pray daily for forgiveness.
This is what John warns us in his epistle: If we say that we have no sin, we deceive ourselves and the truth is not in us; but if we confess our sins, the Lord is faithful and just and will forgive us. In his epistle he combines two things, both that we ought to beg for mercy because of our sins and that we will receive forgiveness when we ask for it. This is why he says that the Lord is faithful to forgive sins, keeping faith with what he promised; because he who taught us to pray for our debts and sins has promised that his fatherly mercy and pardon will follow.