The xtian activists definitely are aware of this overall trend (even if many of them will outright lie about it and many of the flock probably still think they are some kind of supermajority even if they have been losing adherents at about 1% every year for year after year) and it’s exactly why they are agitating to fundamentally change this country to a xtian one.
They want to be able to COMPEL people to join/stay in their little book club. The only difference between xtian radicals and Islamists is where the retconning leaves off is different. Both of them worship the same god of “the” bible - Allah/Yahweh/Jehovah and both of them have the same dim view of unbelievers and women and outsiders, etc…