ONLY God can forgive sin.
John 20:23 If you forgive anyone's sins, their sins are forgiven; if you withhold forgiveness from anyone, it is withheld.
Matthew 16:19
I will give you the keys of the kingdom of heaven. Whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven."
So, God forgives us our sins in the Sacrament of Reconciliation, through the words of His priests.