The cross of Christ is the power over sin...by crucifying the outer man sin nature...to liberate the inner man to be joined to Christ in resurrection life. That is the power of the gospel.
To be joined to Christ in resurrection life?
You must be birthed in the Spirit , for God to forgive your sins..much more to it than that....Jesus paid the price for our sins at Calvary....hence being birthed by the Holy Spirit...who came after Jesus death.
Ephesians 2:1-10
New International Version
Made Alive in Christ
2 As for you, you were dead in your transgressions and sins, 2 in which you used to live when you followed the ways of this worldand of the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient. 3 All of us also lived among them at one time, gratifying the cravings of our flesh[
a] and following its desires and thoughts. Like the rest, we were by nature deserving of wrath. 4 But because of his great love for us,God, who is rich in mercy, 5 made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved. 6 And God raised us up with Christ and seated us with himin the heavenly realms in Christ Jesus, 7 in order that in the coming ages he might show the incomparable riches of his grace,expressed in his kindness to us in Christ Jesus. 8 For it is by graceyou have been saved, through faith—and this is not from yourselves, it is the gift of God— 9 not by works, so that no one can boast. 10 For we are God’s handiwork, created in Christ Jesus to do good works, which God prepared in advance for us to do.