The question of what Jesus paid for on the cross is one of the most debated questions among Christians. I believe wholeheartedly that Jesus intended to take the place of the elect on the cross (all those who have ever lived, and ever will live, who trust in Him alone for salvation).
You can disagree with that and still be a Christian, but you cannot hold that Jesus died for every single sin, except the sin of unbelief. Don’t know if you’ve ever heard that, but that is a seemingly common argument for those who may be classified as Arminians. Maybe you’ve thought that at some point. I know I have wondered if that might be true.
But it cannot be true. That argument says Jesus paid for every sin of every person who will ever live, but the only reason anyone ends up in hell is because they do not trust in Christ. If you trust in Christ, since Jesus paid for all your sins, and since you no longer are committing the sin of unbelief (which he didn’t pay for), you go to heaven. But don’t I deserve God’s wrath for committing the sin of unbelief for a period of my life, even if it was just the first 14 years of my life? Who paid for that sin? Either God lets me into heaven with a bad record left undealt with, or I somehow atone for my own sin of unbelief in order to get to heaven. Either way, it is a false gospel.
No disagreement is harmless, but some are more eternally harmful than others (Galatians 1.8).