Ok it's like this.
Jesus a perfect man (born without original sin) and the Son of God, comes to earth to teach and to atone, one for one, for the original sin if Adam by taking on punishment due to Adam, death.
By doing this he cancels Adams debt and we can all go to heaven.
We'll ignore the fact that Eve's original sin doesn't count (after all she's only a woman)
To believe this scenario you have to believe the Biblical account of the Garden of Eden, and that we are all descended from this one real human being Adam.
Since we know that modern humans evolved from other animals in Africa and that the above story is incorrect, the whole basses of Christianity is flawed.
Or put another way you cannot be a Christian and accept the overwhelming evidence for Evolution.

PP