Christianity is reckoned by most people, I imagine, to be at core a religion of salvation. The defining event is the cross, understood as an act of atonement or redemption, the means by which people are saved. If you are not a Christian you are “lost” or “perishing”. If you become a Christian, you don’t simply convert or join: you admit that you cannot save yourself, that you need a Saviour, therefore you repent of your sins and are saved. You then become part of a community of saved individuals, the church, and are expected to do what other saved people do, until eventually you die and go to heaven.
This has certainly been the overriding paradigm for the modern conservative and evangelical church, and we all naturally assume that it’s biblical. At the heart of the New Testament must be the simple and consistent gospel proclamation: you are a sinner, but the good news is that Jesus died for your sins; so believe in him and be saved from lostness before death and annihilation or worse after death.