“Secularism is expected to decline throughout the 21st century...”

Says who? Could the same not be said about every century? This is clearly false.

Right wing trash does seem to be shitting out kids in record numbers.

That's always been religion's plan: domination through numbers. Breed a bigger flock with each generation.

I can't speak for the US situation, but I don't think it's likely to take hold here in the UK. Christianity, in particular, is something that isn't consistently being passed on from parent to child. A lot of Christians are raising children who grow up to be non-believers.

I think that will be super hard to define since it's much easier to define oneself as "pro-choice" than it is to define "religious".

I have a friend that on our first day of meeting defined himself as "a very spiritual guy".
He grew up catholic, went to catholic school, goes to church, plays / sings in the choir, is sending his daughter to catholic bible classes or whatever you call it, etc.
He and I make fun of religion on a daily basis. Make fun of "thoughts and prayers" people.
He believes in god but still laughs and makes crude jokes about jesus, etc.

He doesn't believe in abortion, but thinks it's none of his business what you do with your life / body. So I would call that pro-choice, cut and dried.

However, would you call him religious?