In the old testament, sins were paid for with the blood of animals. In the new testament, all sins (past, present, future) were paid by the only human to live a sin free life - God in human form, the son, etc. That blood sacrifice was intended to fulfill all requirements of blood sacrifices of old testament and need never be repeated. To avail of that sacrifice on a personal level all one had to do was believe in Him (Jesus) who made the sacrifice. I've read and heard some interesting takes on this too like why he was beaten so badly -- he was literally receiving God's wrath on behalf of mankind.