For the Jews, according to Jesus (Luke 16), they have the law and the prophets. God's requirements and plan of redemption had already been revealed to them. For everyone else, the existence of God has been abundantly evident in His creation.
The apostle Paul explains all this in Romans chapters 1 & 2.
There seem to be hints in the bible about the Lord reaching out in search of believers who will reach back. The question may be ill posed. Not whether all salvation is worked through Christ (this is a biblical sine qua non), but whether all people who meet Christ and believe will do so as a result of what we usually know as the Christian church.
“For everyone else, the existence of God has been abundantly evident in His creation.
The apostle Paul explains all this in Romans chapters 1 & 2.”
==
So, accepting the existence of a generic creator God is what keeps them out of hell?