Who knows if there really is such a decline. It is part of the religious totalitarian world view to see any challenge to their supremacy through the eyes of victimhood. Perhaps Christians move horizontally from church to church. Churches then report declines and then start revivals. The days are gone when congregations stay in one church for generations. I am suspicious of claims by xtians there numbers are in decline, they are under attack, or warring on Christmas exists.
God thinks it's fun to confuse primates. Larsen's God!