From Eric Kaufman (Shall The Religious Inherit The Earth? 2010) , a demographer who has studied the changes in religious participation extensively:
If you are going to be religious, you may as well go for the whole thing. Otherwise you might as well be secular.
He is not in the least evangelistic here. He is describing the data of what seems to be the world-wide attitude in nearly all cultures (and their religions) going forward. The flanks (not necessarily the fringes) of religious sentiment are growing, both the believers who are gravitating toward stricter sects, and the seculars who have no affiliation, or even contact with religion at all. It is the middle that is hollowed out - the mainstream denominations of the West, for example. Even in Islam, Bosnia used to have many moderate Muslims. They have tended to go one way of the other in the last two generations. Kaufman describes that fundamentalism arises in response to the impression that the mainstream is abandoning the faith for secular preferences.
Bsking noted years ago how few young people she worked with were interested in all the interesting liberal political things some of the churches were doing. They shrugged and reasoned that if that is what they wanted, they would simply join secular liberal causes, that focused more intently on those things, without dragging God into everything.
Interestingly, Kaufman notes that those traditionalist groups, such as Orthodox Jews or Amish or Hutterites, tend to be pro-natalist and have more children, and this is a stronger long-term strategy for growth than evangelism. Mormonism grew for a long time on evangelism, but now there is "churn" of people who grew up LDS leaving, while there is now only a slightly higher number of births per woman. Their growth has leveled. Evangelicals have churn as well, with almost as many leaving as joining. However, they are strongly pronatalist and will continue to have growth because they have more children. Evangelism is apparently a temporary strategy. I will add that while much is made of the explosive growth of the Christian Church in the early centuries, from a few dozen believers at the outset to 6M when it was the dominant religion in the early 4th C. But most of that increase was in the first hundred years, and most of that was in the first twenty years. After that you can go on 2% growth a year, so long as it is sustained. The miracle of compound interest and all that.
Kaufman does note that the two often coincide - the evangelistic sects tend also to be pro-natal. He does also suggest that the hollowing of the center may be slowing, not accelerating, as there are only so many who want to move to either flank. The ones who wanted to leave are already gone.
I admit I listen to such podcasts with two ears, one listening for religious growth information and understanding, the other for cultural and political knowledge. Those overlap but are not identical.