The Humanist Manifesto, written first in 1933, straight out states its goal was to rid public education, and therefore the nation, of Christianity.
These people have never been secretive about their goals. They proclaim them.
It falls apart when they realize the parts they want to keep (including civil behavior, prohibitions against murder and theft, nuclear families - at least in the context of having males help produce children and provide for the family unit) also go out the window with that Christianity. The rise of unemployed young men and their refusal to breed is directly related to their marginalization via the attack on Judeo-Christian values; the left won’t elect to force that on them (they simply won’t “get right”...