Modern western atheism doesn't predate Christianity, it's a response to it. And pre-Christian agnosticism is both not gnostic (duh agnostic) and predates Christianity.

You're creating a weird distinction to try and justify that western atheism is derived from christianity, which doesn't make sense given that the same stances existed well before christianity.

I'm drawing the distinction between two separate intellectual strands that are, evidently, easily confused.

Contemporary western Atheism is observably derived from Christianity. After all today's atheist isn't motivated by pre-Christian philosophers' doubts about the nature of the Hellenic pantheon. Furthermore, Christianity makes metaphysical assertions that are so much stronger than the old Hellenic ones that they aren't even in the same class.