Note that in both events God acts in Christ! "God was in Christ, reconciling"... and also we speak "in Christ's stead." Jesus, according to both His divine and human natures, atoned for the sins of the world by His perfect obedience and His innocent suffering and death.
AND
Jesus, according to both His divine and human natures, delivers the fruits of His atonement through "the word of reconciliation."
The atonement is history. "It is finished," (John 19:30) "once, for all" (Hebrews 10:10) accomplished on Calvary. The delivery is His ongoing work; happening in the "One Holy Christian (catholic) and Apostolic Church." (Nicene Creed)
2) Just as we can admit none of our work into the earning of forgiveness. So also, there is no place for synergism in God's delivery of forgiveness. It "is a gift of God lest any man should boast." (Ephesians 2:8-9)
As soon as we say that God Himself is the one who delivers His forgiveness to us, the natural question is: what God? And the answer is: the same God who earned the forgiveness--Jesus Christ.
It is not as though Jesus handed off the baton to the Holy Spirit upon ascending. The Holy Trinity is indivisible. And while no one can say "'Jesus is Lord' except by the Holy Spirit" (1Corinthians 12:3).
The Holy Spirit remains the Spirit of Christ who is present by virtue of Jesus who sends Him. (Jn 1:7). And where Jesus is, He is present with both natures—entire and intact (unconfused, unchangeable, indivisible and inseparable).
To ascribe the work of bestowing faith to the Spirit alone, divorces the Spirit from the Incarnate Christ. And finds no ongoing relevance or purpose for the risen body of Jesus.
3) But the Biblical witness is clear: God's forgiveness of sins—spoken through a human mouth is inextricably bound up with the incarnation of our Lord, Jesus Christ.
Throughout the Old Testament, forgiveness is prayed for and left to God. The priests never declared God's forgiveness to anyone. But shortly after Jesus' baptism, we see a remarkable change.
Matthew 9, Mark 2 and Luke 5 all report the healing of the paralytic. He is brought to Jesus, who says, "be of good cheer your sins are forgiven." Those who heard Jesus were astonished at this utterance. The man Jesus was saying what had never been heard before! The scribes counted it blasphemy saying, "only God can forgive sins." But Jesus went on to prove that "the Son of Man has power on earth to forgive sins," by saying, "Arise and walk." And "when the multitudes saw it, they marveled, and glorified God, who had given such power (Greek: exousia) unto men." (Mt 9:8)
God begins a new thing: delivering the forgiveness of sins through a man. The Scribes were right in one respect: only God can forgive sins. That is why no one ever was permitted to speak such things before Christ. But now God has become a man and this man—by speaking—forgives sins.
4) The incarnation has brought about something new...and it is permanent. The multitudes marveled not that "God had given such power to Jesus..." ...but that "God had given such power to men." This is the Gospel history which undergirds Jesus' promise to the Church: "I will give you the keys of the kingdom..."
A promise spoken to the disciples in both Mt 16(:19) and 18(:18) and fulfilled after the atonement was "finished" (Jn 19:30) and when Christ first showed His risen body to the gathered apostles. He said, "... as the Father has sent Me, even so I send you. And when He had said this, He breathed on them, and said unto them, Receive ye the Holy Spirit: Whose soever sins ye remit, they are remitted unto them; and whosesoever sins ye retain, they are retained." (Jn 20:21-23) Now the Word sent by the Father is sent also by the Son and is heard wherever those sent by the Son are speaking the absolution.
This is the story of the book of Acts as Paul put it in his sermon at Antioch: "Be it known unto you therefore, men and brethren, that through this Man [Jesus] the forgiveness of sins is preached unto you" (Acts 13:38) And this is still the story of God's forgiving sin as Paul says in (2 Corinthians 5:18): "all things are of God, who hath reconciled us to himself by Jesus Christ, and hath given to us the ministry of reconciliation"
