One of the most powerful effects Christianity has had on Western culture is it has imposed an appreciation of forgiveness as a kind of pinnacle of morality. (Derrida has a line on this in his On forgiveness (Routledge, 2001) if I am not mistaken, something to the effect that the appreciation of forgiveness was the result of the Christianization of the West.) It is Christianity that teaches, For if you forgive other people when they sin against you, your heavenly Father will also forgive you. But if you do not forgive others their sins, your Father will not forgive your sins (Mt 6.14-5).
Forgiveness, however, is a difficult thing, as anyone knows who has been in a position to forgive before. Cyril of Jerusalem motivates us to forgive by bringing to our minds the disproportion between our sins against God and the sins of others against us:
The offenses committed against us are slight and trivial, and easily settled; but those which we have committed against God are great, and need such mercy as His only is. Take heed therefore, lest for the slight and trivial sins against thee thou shut out for thyself forgiveness from God for thy very grievous sins (Lecture XXIII, 16).
When we think about the actual damage done to us by other persons, we find it to be quite minimal: an insult here, a poorly toned response there, etc. Of course sometimes the evils done us are great as well. But what can compare, like Abelard says somewhere, to that sin which put Christ on the cross? How can anything we do compare to the murder of the Son of God himself? And yet for that same murder we are all responsible, since Christ died for all (2 Cor 5.14) -- and yet to this very "you" who crucified Christ (Acts 2.22-3), the Holy Spirit is promised (vv. 36-9). If Christ can forgive us this, then what are we allowed not to forgive?
Now there is debate whether or not forgiveness is unconditional, and whether Christians are expected to forgive unconditionally. Some persons point to the alleged fact that God forgives conditionally as evidence that Christians are not expected to go beyond this, but I don't agree with the premise. I think God forgives first, and because he forgives us he saves us and atones for us. Thus Paul says that God was reconciling the world to himself in Christ, not counting people's sins against them (2 Cor 5.19). The forgiveness is first and then the reconciliation on the basis of the former. More than that, unconditional forgiveness is ostensibly the attitude of the person who is full of the Holy Spirit, who receives that merciful heart of which St. Isaac the Syrian spoke. Consider Stephen at Acts 7.60, who prays to God that he not hold the sin of his murderers against them: he doesn't wait for their repentance but prays for them while they are in the act of sinning! He does this because he is full of the Holy Spirit (Acts 6.5).
Forgiveness, therefore, is a quality which Christians must seek to embody day by day, hour by hour. We have to be transformed and changed into the likeness of God who is quick to forgive, and we can only accomplish this through the presence of the Holy Spirit within us. When we are full of love, forgiveness flows naturally and easily.