Who was the first atheist?

Answer 1

Religious belief dates back to pre-history and, so, presumably, does non-belief.

In Europe atheists could (and were) prosecuted - in Britain, being an atheist used to be a capital offence - so undoubtedly, many atheists simply kept quiet about their lack of belief.

Probably the first person to openly admit to Atheism was Paul Baron d'Holbach in the late 18th century.

Answer 2

It is difficult to pinpoint the first atheist as disbelief in deities and questioning of established religions have been documented throughout history. Some ancient Greek philosophers, like Diagoras of Melos and Critias, are considered early examples of atheism.