One of the greatest truths preached by Christianity is the forgiveness of sins. If one claims to be a Christian and then doesn't forgive another's sins, then indeed that one is not much of a Christian.
That is why those who compiled the Creed put it in: "I believe in the foregiveness of sins." It is easy to forget unless we are reminded. And much harder to put into practice.
See, even sound doctrine can be manipulated to serve sin when we allow ourselves to ever stop remembering we serve a holy God. We are to seek first His Kingdom and His righteousness. Forgiveness is about restoration for the sake of righteousness. It isn't to give aid and comfort to unrighteousness. We have lost our ability to reason out these matters. Forgive in a way that honors God's righteousness. That means you do not keep a meth using, prostitute seeking pastor in leadership even if he is sorry. If he repents, you keep him in fellowship but you watch him so that you can help REALLY restore him. If he doesn't repent you break fellowship with him, hoping it will help lead him to eventual repentance, and remembering we serve a HOLY God. There are others in the fellowship who could stumble because of this so you must be very careful. You have to look at the big picture and remember who you serve.