Simply put, 93 million children are born into non-Christian homes, and 45 million are born into Christian ones. Aside from the births, Christianity gets 15 million converts and loses 12 million defectors per year. Yes, “evangelicals” (a subset of Christianity) is growing at a faster rate (new members per 100 old members)–so Christianity is growing more evangelical.
BUT–evangelicals are not gaining more than 3 million nett per year.
At most Christians are gaining 48 million (births + nett converts) while the non-Christian world gains 90 million (births - nett converts).
Those new non-Christian babies will generally only be evangelized from the outside. Yet the vast majority of them do not know believers who will evangelize them.
This is not a recipe for closure.
If anyone is going to change it, it requires getting out of our Christian circles and into non-Christian circles.