Two verses in the Old Testament show that God is the one who forgives, but this does not mean that He does not (rather has before) used a priest to offer petition for sins. This especially demonstrates that this is in no way contradictory.
Old Covenant: psalm 103 (God forgives ) Leviticus 19 : 21-22
21 but he shall bring a guilt-offering for himself to the Lord, at the entrance of the tent of meeting, a ram as guilt-offering. 22 And the priest shall make atonement for him with the ram of guilt-offering before the Lord for his sin that he committed; and the sin he committed shall be forgiven him.
But that is the Old Covenant, of course:
New Covenant:
jn 20: 21 - 23
21 Jesus said to them again, ‘Peace be with you. As the Father has sent me, so I send you.’ 22 When he had said this, he breathed on them and said to them, ‘Receive the Holy Spirit. 23 If you forgive the sins of any, they are forgiven them; if you retain the sins of any, they are retained.’
acts 19 : 18
Many of those who believed now came and openly confessed what they had done.
2 corin 2 : 10
Anyone you forgive, I also forgive. And what I have forgiven--if there was anything to forgive--I have forgiven in the person of Christ for your sake,
2 corin 5 : 17-21
17 So if anyone is in Christ, there is a new creation: everything old has passed away; see, everything has become new! 18 All this is from God, who reconciled us to himself through Christ, and has given us the ministry of reconciliation; 19 that is, in Christ God was reconciling the world to himself,[a] not counting their trespasses against them, and entrusting the message of reconciliation to us. 20 So we are ambassadors for Christ, since God is making his appeal through us; we entreat you on behalf of Christ, be reconciled to God. 21 For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.
Clearly, Jesus came to forgive sins
During His public ministry, Jesus preached about the forgiveness of sins: remember the parables of the Prodigal Son (Lk 15:11) or the Lost Sheep (Lk 15:1), and His teaching that "There will likewise be more joy in heaven over one repentant sinner than over 96 righteous people who have no need to repent." (Lk 15:7) Jesus Himself forgave sins: remember the story of the woman caught in adultery (Jn 8:1) or the woman who washed His feet with her tears. (Lk 7:36) He also taught us to pray for forgiveness in the "Our Father."
His mission of reconciliation would climax in His passion, death and resurrection: Jesus suffered, died and rose to free us from sin and death.
On the first Easter Sunday evening, Jesus appeared to His Apostles, "breathed on them," and said, "Receive the Holy Spirit. If you forgive men's sins, they are forgiven them; if you hold them bound, they are held bound." (Jn 20:21-23)
Jesus called the sinner to realize the sin, to repent of it, and to be reconciled with God and neighbor.
God and neighbor. Sin is not only against God, Himself, but against His creation. We must ask for forgiveness from God, but also from our neighbor, and we do this through the ministry of reconciliation (2 Corin 5) with a priest who is in the person of Christ (2 Corin 2).
Christ is in the Church, the priest is in the person of Christ, so therefore all sin is against God and His Church, and severe sins must be forgiven through the instrument of a priest before God as they are mortal sins:
1 John 5 : 16 -17
16 If you see your brother or sister[a] committing what is not a mortal sin, you will ask, and God[b] will give life to such a one—to those whose sin is not mortal. There is sin that is mortal; I do not say that you should pray about that. 17 All wrongdoing is sin, but there is sin that is not mortal.
