Don't forget much of that population is young yet, and going through a phase of rejection of religion. They tend to come back into the fold as they age.<quoted text>
That's 0.0005% of the population of the the world converting. Not a very fast growth, it's slowing down in more educated locales, and increasing a lot in places like Africa. Which does not bode well for you, because a starving person will convert to any religion for a slice of bread.
Over a sixth is between 20-29.