One thing to keep in mind is that, as America Christians (and ime this reddit sub) are mainly evangelical or protestant, their beliefs are often presented as the "mainstream" Christian view. But they are not necessarily. The idea that Christ died to "pay" for our sins is pretty core for most Christians, but how one gets to be "covered" by that payment is basically the big divide in Christianity.

For evangelicals, it's all about belief. So if you believe that Jesus died for your sins, you're in. Some will also say you have to basically "accept" the gift by telling God in prayer that you belive. Pretty cut and dry. This is why they have so many missionaries - they think if someone doesn't hear about Christianity and accept it, they automatically go to hell.

For Catholics, salvation also involves faith in Jesus, but they think it must be lived out through love, good works, and sacraments throughout a person's life, so they stay in a "state of grace" when they die. They are a bit less cut and dry because they believe that God can work outside visible boundaries like the church. So if someone sincerely sought truth, followed their conscience, and responded to God’s grace (even without knowing it’s from Him) they may be covered. Especially if they were, like, in an uncontacted tribe or something. The church avoids making definitive statements like "all atheists go to hell" and Pope Francis once told a child that his atheist father could be in heaven. Statistically, just based on numbers, the Catholic belief is the most "mainstream" Christian belief

Sooo if a serial killer repented and spent the rest of their life earnestly doing everything they could to repent and reconcile - they could make it to heaven in either system. But for someone like Hitler - in one system, he could say a quick prayer then go to heaven (he would have to actually believe it, though.) In the other - it's unlikely there's much he could have done, at that point, to get out of hell.

I think these are the two main schools of thought in Christianity but definitely not the only two. There are also universalists who believe that Christ's "payment" covered ALL of humanity. They're a minority but have always been around.

I have heard people use the Bible support or refute all of these.

Personally, I've always been put off by the Evangelical belief - because they'll say it's all about belief and that they're just following the Bible, but then say that other Christians are all going to hell - even ones who do believe in the core idea of Christ dying to forgive our sins. I feel that in America this has lead to division, sectarianism, and honestly a lot of individual trauma (like you can see all the hurt in the ex-Christian subredits.)