Are there fewer Christians, or are there just fewer who claim to be Christian?
I think the church may be going through a “cleansing” as well as a falling away as prophesied in 1 Thessalonians.
The wheat from the tares.
There are fewer who claim affiliation with any specific denomination; they no longer see the often politicized and the stringently dogmatic denominations as a source of guidance or reinforcement of their faith. The situation today seems to be similar to the conditions at the beginning of the reformation. This does not mean they have abandoned their Christian faith.
If this is indeed “The Great Falling Away” that the Bible speaks of, then it is a sign that Jesus will return soon. Christians pray every day, “ Even so, come quickly, Lord Jesus!”
....Then a needed rebuilding.