|A 1991 citation (Ѻ) by F.F. Centore, referring to Arthur Schopenhauer, the Goethe-trained assassin of god (Ѻ), intellectual mentor to “god is dead” declarer Friedrich Nietzsche, as “first true atheist” of the world.|
Greek thinker Diagoras (c.448-388BC), aka “Diagoras ‘the Atheist’ of Melos”, a disciple of Democritus (Ѻ), cited by Cicero, among others, sometimes referred to, in the history of atheism (Ѻ), as the “first atheist” or history's earliest known “confirmed atheist”, as some (Ѻ) describe him, or the “first true atheist” in Greece.  Athenagoras, in his A Plea for the Christians (200AD), refers to Diagoras as follows:
“With reason did the Athenians adjudge Diagoras guilty of atheism, in that he not only divulged the Orphic doctrine, and published the mysteries of Eleusis and of the Cabiri, and chopped up the wooden statue of Hercules to boil his turnips, but openly declared that there was no God at all.”
The reason, supposedly, that Diagoras, was led to the supposition that god does not exist, was the result of frustration by the fact that a perjurer was not punished by the gods. 
French closet extreme atheist priest Jean Meslier (1664-1729) is described by Michel Onfray (2005) as the start of true atheism: 
“Meslier’s Testament appeared in 1729, after his death. He had spent the greater part of his life working on it. The history of true atheism had begun.”
Onfray (2009) elaborated rather cogently on this distinction as follows: 
“Meslier’s war cry [Testament, 1729], never before heard in the history of western thought, offers one of the first true atheist moments, if not the first. Prior to him, they call the agnostic an atheist who, as Protagoras, concludes that when it comes to god one can conclude nothing; the pantheist who, such as Spinoza, affirms its existence consubstantial with nature; the polytheist, like Epicurus, who teaches its multiplicity; the deist, in the way of Voltaire, for whom god creates the world en bloc, but does not care about the details; or whoever’s idol does not correspond to the strict criteria established by the church. Now, the atheist clearly says that ‘god does not exist’. This is what Meslier clearly writes: ‘there is no god’ (chapters 59, 74, 93, 94)—that is clear and distinct, blunt, and straightforward.”
There seems to be some validity to this statement, being that Meslier was very influential to firstly Voltaire (1694-1778) (see: Voltaire on religion), who charged the French enlightenment with his work; secondly to Denis Diderot (1713-1784), who viewed the Testament as a kind of “underground bible”, moving about in the literary underground, and who in his The Promenade of the Skeptic (1747), likened Meslier to an “intellectual guerrilla fighter” (Ѻ); and thirdly to Baron d’Holbach (1723-1789), a top extreme atheists, known widely known as the “Newton of the atheists” (Ѻ), whose 1770 The System of Nature: the Laws of Moral and Physical World, became known as the “Atheist’s Bible”.
French encyclopedist Denis Diderot (1713-1784), a Haught disbeliever (#11), is referred to by Michael Buckley (1987), among others, as having been history’s “first true atheist”; others have labeled him as “France’s first true atheist” (Ѻ). Diderot's claim to atheism fame, according to Buckley, is his 1749 Letters of the Blind, wherein we find:
“If you want me to believe in god, you must make me touch him.”— Denis Diderot (1749), portrayal of fictional conversation of Nicholas Saunderson with a priest, in Letters of the Blind (Ѻ)
Buckley asserts that this publication “introduces a critical transition in Western thought with its dismissal of transcendence and assertion of the virtualities of dynamic matter.”  In response to Voltaire, who wrote to Diderot to comment upon Letter on the Blind, Voltaire defended the concept of god, to which Diderot replied that he was a believer: 
“I believe in god, although I live very happily with atheists. It is very important not to mistake hemlock for parsley; but not at all so to believe or not in god.”
and hence not an atheist, let alone a true atheist.
Some refer to Johann Fichte (1762-1814) as the “first true atheist”. (Ѻ)
German thinker Arthur Schopenhauer (1788-1860), a Haught disbeliever (#21), was famously referred to by Friedrich Nietzsche, in his Gay Science (1882), as Germany’s “first true atheist”, as his view (Ѻ) has been retrospectively summarized. The specific statement by Nietzsche is as follows: 
“Schopenhauer was the first admitted and inexorable atheist among us Germans.”
Other related views are as follows:
“Schopenhauer prided himself on being the first true atheist in German philosophy, and scorned his contemporaries’ attempts to substitute a world spirit for a bankrupt deity. Yet he never abandoned a notion of cosmic justice.”— Susan Neiman (2004), Evil in Modern Thought: an Alternative History of Philosophy (Ѻ)
● Atheism timeline
1. Onfray, Michael. (2009). “The War Song of an Atheist Priest”, in: Testament: Memoir of the Thoughts and Sentiments of Jean Meslier (translator: Michael Shreve) (Preface, pgs. 17-24; quote, pg. 19). Prometheus Books.
2. (a) Buckley, Michael J. (1987). At the Origin of Modern Atheism (pg. 222). Yale University Press.
(b) King, Mike. (2007). Secularism: the Hidden Origins of Disbelief (pg. 142). Casemate Publishers.
3. (a) Nietzsche, Friedrich. (1882). The Gay Science (pg. 357). Publishers.
(b) Berman, David. (2009). “Schopenhauer, Nietzsche, and Atheism”, in: Encyclopedia of Psychology and Religion: L-Z (editors: David Leeming, Kathryn Madden, Stanton Marlan) (pgs. 823-25). Springer.
4. Herrick, Jim. (1985). Against the Faith (pg. 75). Prometheus Books.
5. Versnel, H.S. (1981). Faith, Hope, and Worship: Aspects of Religious Mentality in the Ancient World (pg. 40). Brill Archive.
6. Onfray, Michel. (2005). Atheist Manifesto: the Case Against Christianity, Judaism, and Islam (Traité d'athéologie: Physique de la métaphysique) (translator: Jeremy Leggatt) (pg. #). Arcade Publishing.