Why do Christians become Christians?

[Mister Agenda]

So, how large does a pool have to be before hundreds of millions of people are not an exception? 

Okay, say we take the US statistic of 85% of Christians were born into a Christian home. You are claiming that these people only stay because of the "brainwashing" they received as children. The 15% adult conversions are exceptions. You are also concluding that the message that Christianity is a con (or however you want to characterize it) and science can prove it is not working on (319 million US population x 70.6% Christians = 225 million x 85% born into Christian homes) = 191 million people. And this is in the wealthiest country with cast amounts of information available to anyone with access to a computer (to say nothing about the education system's attempt to undermine faith). 

I don't think the numbers support your conclusion.

You don't even know what it's a pool of. You pointed out yourself that a lot of Nones are just unaffiliated generic Christians. 'Only went to church several times a year as a child' isn't the same thing as 'raised without exposure to Christianity'. Frankly, exposure at a young age is almost impossible to avoid in a country with such a large majority of Christians.

And you're losing Christians faster than you're replacing them, except in developing countries, where Islam is also rapidly increasing, and incidentally, where Christians tend to have large families, too.

We can't know if 'being drawn to Christianity' is significant if we don't have much understanding of the pool they're drawn from.