Christ Jesus, not the Law, is revealed in the Gospels as the one Mediator between God and sinners. Christ, the God-man, communes with sinners who remain sinners their whole life long but now through faith, for Christ's sake, have no sin credited to them.
Heaven, it turns out, depends upon Christ taking our sin (indeed "becoming sin for us" as St. Paul writes in 2 Corinthians 5:21), because in the end, either that sin will be on Christ, or us. If Christ is innocent and does not carry our sin, then we carry our sin and will die and be damned by it. Our sin must become Christ's own sin, or we will perish eternally. The word "sin" then, according to the apostle, is now synonymous with "Christ" in such a way that Christ is made to be sin for us.
When we say sin, we mean Christ. There's no other way. Our redemption requires the exchange of Christ's innocence for our sin. For example, when Thomas sees the resurrected Lord, he sees Jesus bearing his sin. Thomas sees the marks in Jesus' hands, feet, and side which makes Thomas' sin that Jesus bears for him touchable. Why would Christ take Thomas’ sins into His own flesh? For the same reason it's so important for us to hear the truth about Jesus’ body and blood preached week after week, and not just colorful descriptions of God’s love for us. Christ Jesus takes sin onto His body to take the sin from our own body. We can only know Christ accurately when He's wrapped in our sin. Otherwise, we'll bear them (often with a great deal of denial) until our body eventually succumbs to their attacks.
Jesus knows why His becoming sin for us is so hard to believe. It’s the same reason the resurrection is hard to believe. It's not that people don't believe God can remove sin. What they don’t believe, is that the way He does it (and the resurrection that comes with it) is through the man who is our God, Jesus. "I am the Resurrection," says Jesus, not an abstract miracle or idea (John 11:25).
I am the Resurrection,’ says Jesus, not an abstract miracle or idea
The resurrected Christ is still the crucified Christ, but as Thomas learns, the sins that were his have somehow ended up in Christ's own body. Yet when they're on Christ Jesus, instead of festering unto death, sin is defeated. In Christ, our sin is now "governed," as Martin Luther says. Christ Jesus is now sin's Governor, instead of sin governing us.
Now that our sin is taken from us at Golgotha and made Christ's sin, there is new freedom to announce this good news to everyone. Our new life as "Christian" and as "baptized sinner" is lived in a joyous dichotomy. We struggle against the sin that governs us, but we struggle in hope against sin because we believe in spite of ourselves, what we call "our sin" is now "Christ's sin." This is because "it is no longer I who live, but Christ who lives in me" (Gal. 2:20).
Christ, the God-man, communes with us sinners who remain sinners (in our flesh) our whole life long. But now through faith, for Christ's sake, we have no sin credited to us by God. Christ is made to be our sin that we may be His righteousness, today and always.