Xenophanes (ca. 570 - 478 BC) was an early Greek philosopher who is most famous for his criticism of traditional religious stories. But if you focus too much on his criticisms, you might draw the wrong conclusion: you might mistakenly believe that Xenophanes was an atheist who saw himself as supporting atheism.
That’s just not true. Atheism was rare in early Greek philosophy (and, frankly, for all of antiquity), but criticism of the traditional conception of the divine was still prevalent. That’s due to Xenophanes’ influence.
Check out also Tim Whitmarsh’s book Battling The God’s: Atheism in Ancient world!