How can a Priest forgive Sin?
It is not the priest but Jesus Christ who forgives your sins in Confession.  Christ has given him the power to do it.  On the first Easter Sunday he said to Peter and Apostles: “Peace I leave you, my peace I give you.  As the Father has sent me I also send you: whose sins you shall forgive they are forgiven them, and whose sins you shall retain they are retained”. (John 20:23).  If God used a human body to save mankind by dying on the cross, why cannot he use a human person like a priest to forgive our sins?

The power which Christ gave to the Apostles on the first Easter Sunday was not to cease with their death, because Christ knew that there would always be sinners in the Church and a need for forgiveness.  This pardoning power was to be permanent, lasting until the end of time, “Teaching them to observe all things whatsoever I have commanded you: behold I am with you all days even to the end of the world”.  (Matt.  28:20).