Religion will in the long term grow in the USA. Immigration continues, most of immigrants are Catholic (all of Central and South America is predominantly Catholic)
Secondly Americans are having less children. This will leave a greater percentage of children in the future being born into conservative religious families whose religion emphasizes and encourages larger families. (Orthodox Jews, conservative Muslims,
Mormons, those who identify with the conservative wing of the Catholic Church, more conservative Evangelicals and others)
Those raised in these families will be more likely to be religious themselves and also marry and have more children. Meanwhile the more secular Americans are having less children. One type of essay that seems to crop up in my Medium feed quite often is written by a young non religious women explaining that she has chosen not to have children. I work in real estate in NYC and I work with Orthodox Jews they all either have children or plan to.