system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What "sins" did Jesus "die for" exactly? What would have supposedly happened to us if he didn't? (Serious)
gpt
Christians believe that after Adam ate from the tree of knowledge of good and evil, that all humanity was fallen and carried a sin nature, even the most innocent of babies carries this with them (passed from father to their children).  It's this nature that causes sin to be tempting.  The only way to pay for sins is via blood sacrifice (thus the reason Hebrew law required all sorts of animal sacrifice).  The best analogy I've heard for a sin nature is bamboo which grows sprouts rapidly and spreads widely.  The sprouts can be cut but the root remains and spreads.  

Jesus being born with God as his father, lacked this sin nature and offered a way to undo the curse brought on by Adam's sin.  Because he didn't succomb to temptation, and lead a sinless life, his death becomes an eternal sacrifice for all who accept it.  By becoming a Christian, Christains believe that they have the keys to victory over sin (not that they'll remove the root, but can machete the top effectively, and eventually the root will wither and sprouts will be less frequent).  

That's not really ELI5, but if you have questions I'll try to explain more clearly.