You can pull back even further and ask the simpler question "If Christianity had been introduced today and its ideas was brand new to everyone, who would voluntarily convert?"

Social, cultural, and institutional inertia are the primary driving force behind Christianity's survival. It's there because people are used to it. The vast majority of their defenses of the faith are based on values and assumptions they would never, ever apply elsewhere.

The God of the Bible is a complete asshole and one-dimensional villain, full stop. Imagine Marvel introduced a new villain whose goal was to gain unlimited power and force people to worship him or be condemned to eternal torture after they died. Audiences wouldn't be debating the morality of his actions, as they did with Thanos. They would call him a comic book villain.

99% of Christians have not read the Bible, which is hilariously obvious because the vast majority of things they get wrong or fail to notice are on the first couple pages. God accidentally creates the universe twice in a row on Page 2 and nobody mentions this. Sodom and Gammorah never references homosexuality. Noah fucks his daughters. And so on.

It's a moral and intellectual mess whose continued existence has very little to do with merit.