The explanation I’ve been given is actually kinda cool in a, like, fantasy worldbuilding sort of way. By nature God is perfection, and since Adam and Eve fell short of perfection, mankind is tainted by imperfection. That falling short is sin. (There is some debate about whether God “decides” what is sin, or whether it is some sort of objective nature of God, which I also think is sort of fascinating.) Man needed ways to be absolved from sin to be in communion with God. This largely involved sacrificing animals to cover over sin with blood, and this symbolism is all over the old testament.

Christianity posits that Jesus was both God and Man, combining the two natures. He lived a sinless life, but still faced death (which shouldn’t happen to someone without sin). This causes a sort of divide-by-zero error that cancels out the sin of all who are linked to Jesus through the sacraments (especially baptism and communion). This allows humans to be “in communion with God” without continued animal sacrifice. And now that living a perfect life, which is impossible, is not necessary, Christians should do good things in the world to reflect the grace they’ve been given.

Unfortunately, most Christians know fuck all about their own religion and don’t understand it at a fundamental level, and think it’s their job to go around judging people and telling them to repent.