It is a simplification, but broadly correct, to regard intellectual anti-Christianity as the root and basis of Leftism.
In other words, Leftism is what you get when anti-Christianitry is rationalized and systemized.
Because the desire of secular materialists is not to refute Christianity (refutation of something rational has only a temporary effect) but to discard Christianity permanently.
Not to out-argue Christianity but to set it aside without argument.
And to do this by changing the subject for mankind - by changing the nature of public discourse such that Christian claims are rendered un-discussable.
This has been achieved through the past half century; but at terrible cost: the cost of destroying reason and putting metaphysical assumptions off-limits.
Error is now entrenched and intractable in the public arena.
Professional discourse is bureaucratized.
Truth talk now is to be found only by active seekers, rarely, in private; and mostly in books not people.