I've seen a couple of answers in this site which explained the meaning of "Jesus died for our sins". In these answers I gathered that Jesus (a representation of God himself) died so we could be without sin. Specifically this bit:
"Jesus died for our sins in that He paid the penalty for our sins. As you point out, this does not mean we stop sinning. What it does mean is that the penalty incurred by our sins is satisfied by the payment made by Jesus Himself."
And this quote:
"For God so loved the world that he gave his only Son, so that everyone who believes in him might not perish but might have eternal life. For God did not send his Son into the world to condemn the world, but that the world might be saved through him. Whoever believes in him will not be condemned, but whoever does not believe has already been condemned, because he has not believed in the name of the only Son of God. "
It sounds like the sacrifice of Jesus unlocked eternal life (=heaven) for us all, and that now we can actively seek to deserve it. What happened to everyone that lived before this sacrifice? It seems like they couldn't have been saved before this event. Assuming there were people who lead a good life in this lot, were they transferred somehow?