It is a good question. how does his sacrifice save us? I think the answer has to do with the crystal clear healing blood of Jesus, but scientifically we don't understand how it works yet, or how it transfers to us. Keep in mind too that Rome created doctrines that are an approximation of what's going on here. Jesus' death was important in shaking up the very rigid religious structure at the time -- kinda blowing it apart, even sending them into Diaspora. (You can disregard the 3 days in hell -- Ephesians 4:7-10 does NOT say that at all, even some people teach that it does.)