A sin is anything that is against God’s will. As we are not God, anything we do could theoretically be considered sin. This is encoded in the idea of original sin. Humans are just bound to not be God. According to Christian theology, Jesus Christ forgave us all for our sins, now and into the future. This includes original sin, and any new sins we commit.
God is not interested in perfection, He is already perfect, he doesn’t need us to be. It’s us that chases the idea of perfection, God’s just happy to help us along the way.
Heaven and hell are the only two places that souls can go eternally after humans die. They are defined by proximity to God. Christians do not believe in alternative afterlives. You either wind up in heaven, or you wind up in hell. Both heaven and hell are graduated, the graveness of your sin and your degree of lack of repentance determines which level of Hell you go to if that’s where you land.
Bear in mind that all sins can be forgiven and that the only sin which can’t be forgiven is when you don’t want forgiveness.
There is no sin in heaven because God is so strong there that it’s literally impossible to sin. You can’t take up arms against your fellow souls, your weapons just wouldn’t work. We are here on Earth to be tested, once we pass the test and get to heaven the test is over.
Catholics believe in something called purgatory, where you repent for the sins you couldn’t repent for in life. Remember this all revolves around your willingness to ask God for forgiveness. Most souls spend some time in purgatory before finally gaining admittance to heaven.
I’m curious to see what Catholics come up with for atheists and for people like myself who do not follow Christian mythology. I suspect they’ll just leave it at “your relationship with God is yours and yours alone.”
Much is made of the judgment God passes on you when you die, but from God’s perspective it all operates simply on the will of the person. If they don’t want to be close to God, then they won’t be close to God. Christians have deemed this Hell but that’s just like, their opinions, man.
I imagine there could be a kind of “indefinite purgatory” set aside for atheists. They sit there until they decide they want to go to heaven or hell. This idea would be a bit dangerous to Christians because it really threatens their binary afterlife. But I can easily see how God might see atheism as a failure of Christians to do their best work, so he’ll give atheists a second chance to, with the benefit of eternity, to decide whether to accept God or not.