It's based on the old idea of offering sacrifices to atone for sins. Do bad thing, sacrifice a dove or whatever to God to make up for it.

The idea is that God decided to do away with the sacrifice system using said system, by sending and then accepting a sacrifice great and pure enough to wipe the slate clean forevermore - his own self/son.

I've heard that it hits people from cultures where they do still sacrifice for every sin particularly hard - we might not have the frame of reference to really get this fully anymore.

how does Jesus getting crucified forgive my sins? Is it some sort of ancient Christian bar bet?

Speaking as someone literally brought up in a cult like environment, it's just one of the many nonsense word-salad doctrines that people live by when those people were never able to separate their feelings from their world. IE: there is a segment of the population who do not have a distinction between an outside world, separate from their feelings about it.

This is a reflection of how the brain works at a most basic level. It's not a logic tool for reasoning out problems, not by default at least. It's default instruction state is to assemble experiences and associations to write a story to explain how you feel, and it doesn't actually have objective understanding about the world, so those stories do not need to make sense.

When you really, truly internalize and digest this fact, you will understand so much about yourself and others. You can overcome some depressive episodes and know how to make people like you, how to manage addiction and unhealthy behavior and how to avoid being manipulated by others, and so much more. It's vastly important we understand this about our brains.

You have to actually train your brain to actually analyze and understand the world around you in a way that shows you how you and the world relate to each other. Most people don't do this work, but brains are good enough at taking advantage of your environment that they can still get through life... but it leaves a lot of room for huge errors in reasoning. In fact, it's not conscious reasoning at all, it's story-building followed by total acceptance of this story without question because you think it's you reasoning, but it's just how your brain weaves narratives in your mind.

So for the people who never learned this distinction, they just feel a thing, and then either let their brains assemble a story to explain it, or they latch onto someone else's supplied story. This is how people are manipulated on mass scales.

"Jesus died for your sins" makes no logical sense, but it's not meant to, it's meant to make you feel like something is being done about the thing you worry most about, if you're going to see your loved ones again in heaven. That's a paralyzing fear for almost every human who's ever lived. Our awareness of death has opened a huge vulnerability in our reasoning skills and caused us more death and harm than if we didn't worry about it so much.

Once you have a McGuffin that makes you feel protected from this thing you fear most, you are more likely to reinforce and build further narratives around this idea to protect it. To not protect it, to dismantle it and try to figure it out is literally painful to many people, because it invites in the question... What if you're wrong?" and even approaching that question makes people who have never processed these emotions absolutely fall apart.

edit: I want to add one thing, that the more you think about the really hard thing, your inevitable end, it becomes easier to accept and make peace with. Especially as you get older and more aware of your own limitations and realize you're kinda stuck on rails in this life. There is no bigger story or experience you will miss out on.

Yeah it would have been way easier for the apparently all good, all knowing, all powerful god to just, you know, forgive us, but that wouldn't have made for a good book

Well I am an atheist myself, but the key is that in the old testament god gave every human the blame for the original sin.

And the special thing with Jesus dying and taking all sins with him is, that now god won't hate every human just because Adam and Eve stole some fruits.

This some weird story but yeah. Basically after Jesus, god wasn't that angry anymore.