He who believes in him is not condemned; he who does not believe is condemned already, because he has not believed in the name of the only Son of God.Jesus here is calling unbelief a sin. Then there is Rom 1:18-23:
For the wrath of God is revealed from heaven against all ungodliness and wickedness of men who by their wickedness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made. So they are without excuse; for although they knew God they did not honor him as God or give thanks to him, but they became futile in their thinking and their senseless minds were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man or birds or animals or reptiles.So Paul is saying that much of the time people know on some level some truth about God that they are refusing to believe. There are two questions to think of here. Is atheism itself immoral? And are atheists fully culpable for that immoral act? The first answer seems like a clear Yes. The second answer can be different for each atheist. The two factors are freedom and ignorance. How free they were to make the choice they made? Was there intense pressure? Secondly, was their invincible ignorance? That is a lack of knowledge that could not be easily overcome because of strong bias they got from their culture.
The church says we should not judge these questions of ignorance and freedom. They require a heart knowledge of the atheist in question that we cannot have. So we should leave that judgement to God. When the person in question is ourselves we should always assume we are fully culpable. We might not be but we should not use that as an excuse.
What happens is that some Catholics judge atheists and say they are going to hell. Then some other Catholics react to that mistake and judge atheists the other way. They assure them they are going to heaven. Both are errors. We need to tell atheists that their unbelief might well cost them their eternal soul but we cannot know that.
Anyway, that is not the really interesting question. What I was thinking about is where we get the idea that false belief systems are not immoral. We see that having a false belief about sexual morality and acting on it by committing an immoral act would be immoral. Still when we switch the example to having a false believe about theology and acting on it by committing an immoral act then we often don't see that is immoral. We seem to be OK with sins against God. When someone blasphemes God or someone worships someone or something but refuses to worship God. We give people a pass for bad theology but not for bad morality.
I think it goes back to the doctrinal agnosticism we got from the protestant reformation. The slogan was faith alone but the actual content of that faith became a big question mark. At the beginning the different branches would condemn each other to hell. If you believed the wrong thing about the Lord's Supper or baptism you were just lost. But it became impractical after a while. Matters of doctrine that effected faith and worship but did not directly effect morals were kind of relegated to a lower class. Disagreeing about polygamy like the Mormons did was still a big deal. Things like sacraments, liturgy, and the trinity? Those things were no big deal. They could not practically be a big deal because such disagreements were so common.
What does that do? It inverts the commandments. See Mark 12:28-31:
And one of the scribes came up and heard them disputing with one another, and seeing that he answered them well, asked him, “Which commandment is the first of all?” Jesus answered, “The first is, ‘Hear, O Israel: The Lord our God, the Lord is one; and you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength.’ The second is this, ‘You shall love your neighbor as yourself.’ There is no other commandment greater than these.”What we end up with in practice is the second commandment being taken a lot more seriously than the first. Matters involving loving God with all you heart, soul, mind and strength are places where agreeing to disagree reigns supreme. Matters involving loving your neighbor? We don't agree to disagree about those. We need to know what to expect from our fellow citizens from different faith traditions. In practice, for the first few centuries after the reformation the Christian morals about how to treat other people went pretty much unchallenged. Even today much of it is but in the last century we have the exceptions of sexual morality and abortion. So even that is breaking down and will continue to break down.
But this unspoken idea that false beliefs about God are not really a big deal has infected even Catholic thinking. Catholics, of course, don't have nearly as much doctrinal uncertainty. We believe God reveals Himself through the church so matters of pure theology can be resolved definitively. Yet we don't treat them with the same seriousness. We even wonder of there is anything morally wrong with denying that God exists. If you are a nice guy and you pay your taxes and don't cheat on your wife then what is the big deal? Jesus thought it was a big deal or he would not have put loving God as the #1 commandment. St Paul thought it was a big deal or he would not have said "for although they knew God they did not honor him as God or give thanks to him, but they became futile in their thinking and their senseless minds were darkened." If we really believe God belongs at the center then we need to be clear. Getting God wrong is a moral matter. Faith is a virtue. The content of faith matters.