Jonathan Rée discusses the evoluton of atheist thought over the ages in New Humanist:
[Percy Shelley's] pamphlet on “The Necessity of Atheism,” published anonymously in 1811 when he was 18, got him expelled from Oxford and disowned by his family, but he stood by it all the same. He may not have been the first atheist to come out of the closet, but he was the first to flourish the title with bravado and panache. On the other hand there was less to his atheism than meets the eye. “It is a good word of abuse,” he said, and he deployed it to advertise his revulsion from the Christian idea of a god who created the world and established the distinction between good and evil. But strictly speaking he was not so much an atheist as a pagan theist. His denial of God, he explained, “must be understood solely to affect a creative deity,” while the “hypothesis of a pervading spirit co-eternal with the universe remains unshaken.” In reflective moments he preferred to call himself a deist.
If the world’s first celebrity atheist was a deist then the word “atheism” seems to be in trouble. Hence the rise of the term “new atheism” to distinguish atheists who really mean business from those who prefer to hedge their bets. Like “atheism” itself, however, “new atheism” began life with negative connotations. It can be traced back to the 17th century, when it – or rather its French equivalent – was used to alert Christians to the threat of Spinozism. But nouvel athéisme was itself a dark phrase, since Spinoza believed passionately in something called God, though he shocked the orthodox by identifying it with nature as a whole rather than a transcendent supernatural agency. During the 19th century, as Spinoza came to be viewed as a pious mystic rather than a raucous infidel, the “new atheist” tag was transferred first to proponents of the mutability of species, then to Auguste Comte and the positivists, followed by the indomitable secularists Harriet Martineau and George Holyoake, Spencerian evolutionists and Darwinian natural-selectionists, and eventually Friedrich Nietzsche and his enigmatic hero Zarathustra.