"21 He said therefore to them again: Peace be to you. As the Father hath sent me, I also send you. 22 When he had said this, he breathed on them; and he said to them: Receive ye the Holy Ghost. 23 Whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained. (John 20)"
This is one of my burning questions. How are these passages reconciled? My view is that only God can forgive sins, and that the Apostles were granted authority to declare WITH authority what God would do when asked for forgiveness.
Mark 2/Matthew 9 and John 20 do not contradict each other. It is THROUGH the God-man, Jesus Christ, whom Catholic priests base their authority upon, not their own. Catholic priests act "in persona Christi", in the person of Christ, when they forgive sins. The priest offers visibly the healing touch of Christ to the penitent. The priest has been given power to forgive sins as a result of Christ's delegation of authority - but it is based on HIM.
Another reason why this verse is not speaking about merely "declaring what God has already done", notice that Apostles were given the power to BIND or RETAIN one's sins, as well! Look at John 20 more carefully. Christ gives the Apostles the power to retain sins of those who are not contrite. Again, God has delegated His power to the apostles to make such decisions. Through the power given them, they decide to grant God's forgiveness to a particular person.
Regards
On the face of the former, they don't even need to be, as it is the pharisees say that only God can forgive sin. But even taking the word of the Pharisees as inspired in this instance, Christ (God) has placed the Holy Ghost (God) into the apostles and then empowered them to forgive sin. The only reason this is met with any mental resistance in the Protestant world is the extrascriptural anticlerical indoctrination.