Good Question.
Biblical Answer?
(wait for it....)
we're not quite sure.
No really, we're not. Paul's letters suggest that somehow Jesus was a second Adam; that the two (a fall of a former sinless being into death, the death of a still sinless being) somehow mirror each other. The author of Hebrews suggested that Jesus was like a Chief Priest, engaging in a sacrifice for the people - but given that the worth of the sacrifice was infinite, so was the redemption. and so on and so forth. It's an event beyond human capability to understand in it's fullness; a Holy Mystery in many ways.
Sort of like Gravity. We have no idea
why gravity works, or even really
how it works; why mass would automatically be attracted to mass isn't something we have a real clear-cut answer for. But we can observe it, and we know it to be true. Forgiveness and welcome into the Grace and Presence of God is like that. How does it happen? I have no friggin clue. But I know that it does because I have Been There, and the experience is at once more Real and more Ethereal than anything else I have ever encountered.