7 Therefore in this present life, do we ask what comes next, “Forgive us our debts, as we also forgive our debtors.” In Baptism, all debts, that is, all sins, are entirely forgiven us. But because no one can live without sin here below, and if without any great crime which entails separation from the Altar, yet altogether without sins can no one live on this earth, and we can only receive the one Baptism once for all; in this Prayer we hear how we may day by day be washed, that our sins may day by day be forgiven us; but only if we do what follows, “As we also forgive our debtors.”
Accordingly, my Brethren, I advise you, who are in the grace of God my sons, yet my Brethren under that heavenly Father; I advise you, whenever any one offends and sins against you, and comes, and confesses, and asks your pardon, that you do pardon him, and immediately from the heart forgive him; lest ye keep off from your own selves that pardon, which comes from God. For if you forgive not, neither will He forgive you. Therefore it is in this life that we make this petition, for that it is in this life that sins can be forgiven, where they can be done. But in the life to come they are not forgiven, because they are not done.
Source: Sermons on Selected Lessons of the New Testament (New Advent)