Liberalism is the key. When we spend our lives arguing that certain things are just, even if they are not tradition, and we overturn tradition in order to form a more perfect society, then we are already in a mind set of questioning tradition and authority and looking for better answers than our parents.
When you accept the idea that slavery is abominable, then you look at the parts of the Bible that endorse it and go, wow… this book contains either falsehoods or a God that is less moral than I am.
When you accept the idea that gay people shouldn’t be murdered for being gay, you look at the Bible and go… wow. More falsehoods, or an even less moral God.
When you accept that rape is a crime against women, and find that God endorses it and promises the opportunity to rape when you conquer your enemies in the Lord’s name, then you understand that the Bible was written by men who wanted to conquer other human tribes and rape their women, not a holy God.
Even if you accept the idea of a Holy, sinless God, you must reject the Bible as a factual account of that God. And you understand these are just fables, like greek myths, and they are not the source of morality that humanity needs.
Once you’ve let go of the fairy tales, you see there’s no evidence that would lead you to conclude there’s one god, two gods, many gods, or any other supernatural beings. This leads to agnoticism.
More and more people feel this way, but still identify as Christian. But they don’t take the Bible as literal truth and they don’t go to church. They think for themselves, and their answers regarding morality come from human philosophers. They’re secular humanists in all but name.
The less butts in the pews, the better we all are.