Stolen from an old post of mine
In that moment of crucifixion, God took all the sins of the world -- every sin in the past, every sin in the future -- and took them upon himself.
Christians believe that Jesus did this for every one, in a tragically real sense -- he loves you so much that if everyone else in the world was
perfect, free from sin, and had no need for redemption, he'd have gone to the cross anyway, just for you. You didn't ask for it, you might not even
want it, but it's there for you, regardless.
As to why it had to happen, I like St. Anselm of Canterbury's view of "satisfaction." God cannot just wave away sin -- that is acquiescing to evil,
saying that actions and intentions are of no consequence, so evil "wins". So a price must be paid to merit salvation, to meet the justice of
For Anselm, a Middle Ages theologian, different crimes required different levels of satisfaction, based on who they were committed against -- a crime
against the king was far more serious than a crime against a peasant. This is still often the case -- a man who hits another man is viewed differently
than a man who strikes a woman, or a child.
In sinning, we have committed a crime against an infinite being, and have thus incurred a penalty that would require an infinite repayment to meet
satisfaction. A debt that we can obviously never pay, a debt that could never be paid on our behalf by anyone. Except an infinite being.
So, we sin against God, God pays the infinite price to meet satisfaction, and, if we accept it, we are "clothed in Christ's righteousness" when
judgement comes -- Christ, effectively stands up on our behalf, says "I've got this covered", and we're in the pink.