Nietzsche was the most intelligent and sincere Atheist in human history.
He knew Atheism is not a logical conclusion but an axiom or a starting point.
His axiom is quite intriguing (God is dead instead of the usual there's no God).
His personal justification of his axiom is just a sensation of joy (Gay Science) from this axiom, or the joyful knowledge of knowing God is dead.
Also it's very interesting his unique concept of oblivion (logical conclusion of Atheism).
His oblivion is the eternal recurrence/return (his greatest idea according to himself).
This idea says that all things will repeat itself in the same way forever. But there's no awareness of this repetition, maybe only to a few lucky (like him).
Those who are aware of the eternal return must decide if it's heaven or hell. Nietzsche said he decided to chose heaven. He said yes to the eternal return.
This is defined as metaphysical Atheism.
Recently we have a New Atheism proposed by the "Four Horsemen" (Dawkins, Dennett, Harris and Hitchens).
The New Atheism's axiom (PROBABLY there's no God) is justified by Scientism and logical refutation of Sola Scriptura.
Many people give intellectual and volitional value to a normal (but temporary) doubt about the existence of God. This situation is described by the St. John of the Cross as the "dark night of the soul", and he explained this is a necessary purification to the soul.