Religion differs from philosophy in that it is dependent on revelation as opposed to reasoned argument. This is a first approximation; at a second approximation it becomes more subtle.
An argument derived from Epicurus is that religion provides comfort to those who have poor powers of reasoning; and that it is mostly superstition; still when one considers that Christians must examine their conscience to avoid eternal damnation this seems hardly comforting to the Athiests evaporation into nothingness; and when one looks at the internet one observes a great deal of scientific and mathematical mumbo-jumbo ie superstition.
One ought to observe that any form of rationality (and in here I include religion) must also have as a consequence superstition.
A second fallacy, is that people two millennia ago must be worse thinkers than we who are here now, we who in fact are the inheritors of all the knowledge that has been created.
Is it worth pointing out that already in Aristotles Metaphysics he was pondering the existence of the first mover; ie a God in rational terms? And when one considers this, one should recall he is still central to the Western philosophical project; if not only for the sharpness of his intellect, and it's width, but also because he argues as we do today; which seems surprisingly modern, until we realise that we argue as he does: not for nothing was his place of learning was called the Academy.
A third fallacy is that Athiests are more rational than theists. In fact one can prune down their arguments to a core set of beliefs. That is, in Descartes language, they clearly and distinctly perceive that there is no God; thus their statement
why should I prove the existence of something that doesn't exist
This is in fact a mirror image of the theists argument - why should prove the existence of something that I and many others also clearly and distinctly perceive.
In this sense, atheism is a religion; but also it is not as there is not any core rituals, or doctrine as such for it to cohere with; one could suppose, given sufficient time and development that this will or could become a distinct possibility; that until now there has been no significant atheism 'church' - if one discounts, as one should, Comtes Religion of Humanity, and the Brights.
But then again, there has never been a significant fraction of humanity without some form of theistic belief; that there is now, prominently in Western Europe; might suggest that in the future that a coherence into a religious type movement with a positive doctrine might be a distinct possibility.
Finally, one really ought to examine what one means by 'clear and distinct perception'; by what 'light'; and how is this not a 'revealing' and therefore, in a minor way, part of a ladder of 'revelation'.