LONG BEFORE VOLTAIRE...

several ancient Greek philosophers were skeptical of the claims made by Homer and Herodotus about the gods.  Xenophanes (flourished 540-537 BCE) and although we only have fragments of his thought, they are worth posting:

In the well-known fragments B14-16, Xenophanes comments on the general tendency of human beings to conceive of divine beings in human form:

But mortals suppose that gods are born,
wear their own clothers and have a voice and body. (B14)

Ethiopians say that their gods are snub-nosed and black;
Thracians that theirs are are blue-eyed and red-haired. (B16)

B15 adds, probably in a satirical vein, that if horses and oxen had hands and could draw pictures, their gods would look remarkably like horses and oxen.

Xenophanes was neither an agnostic nor an atheist but he is the most important early thinker to question the gods.

Anaxagoras (born ca. 500–480 BCE) may have been the first atheist:

According to Diogenes Laertius, Anaxagoras acquired the nickname Mr. Mind (DK 59 A1); his view that the cosmos is controlled by nous, mind or intelligence, first attracted and then disappointed Socrates (Plato, Phaedo 97b8ff.).

Just as we control our bodies by our thoughts, so the cosmos is controlled by nous; we may be unclear about the details, but the results are obvious to us. One fundamental point about Anaxagoras' theory of mind is that he nowhere in the extant material identifies mind with a divine principle or god.

Thucydides (c.460 - c.407 BCE) was the first true historian because he did not rely on the supernatural to explain events as Herodotus and Homer did. Democritus (c. 460 BCE - c.370 BCE), the student of Leucippus who popularized the atomic view of reality, is certainly the first atheist:

Democritus retains much of the old Ionian and post-Parmenidean interest in nature and natural processes. Where he goes beyond these is in his rejection of any form of divinity either outside of or within nature. As Jaeger says, `His description of nature in terms of the interplay of countless atoms in empty space ruled by the power of chance left no room for teleology and the deification of any moving forces or single primal ground.' II In Democritus, as many have pointed out, the naturalistic implications of Ionian philosophy of nature reach their final and logical conclusion and we have an explicit statement of what, as F. M. Cornford puts it, 'the philosophers call materialsm and religious people call atheism.' SOURCE: A short history of western atheism (1971) by James Thrower, pp. 31-32