Is Atheism an Ism? says:
... source of the errors is basically the same: people who describe atheism as as philosophy, ideology, or something analogous are trying to depict atheism as being much more complicated than it really is. All of these categories are defined in one way or another as systems of beliefs which provide guidance or information. None of this can describe atheism, whether defined broadly as the absence of belief in gods or narrowly as denying the existence of gods.
It's strange that this would happen because almost no one says such things about atheism's "opposite," theism. How many claim that mere theism, which is nothing more than a belief in the existence of at least one god, is all by itself a religion, ideology, philosophy, creed, or worldview? Theism is a common doctrine and it is commonly a part of religious dogmas. It's also commonly part of people's religions, philosophies, and world views. People demonstrate no trouble understanding that theism can be part of these things, but doesn't qualify as one all by itself.
So why do people fail to realize this when it comes to atheism? It's probably because of atheism's long-standing association with anti-clerical movements and dissent from religion. Christian theism has so dominated Western culture, politics, and society that there have been few sources of religious or theistic resistance to this domination. At least since the Enlightenment, then, atheism and atheistic groups have been a primary locus for freethought and dissent from Christian authority and Christian institutions.
What this means is that most people engaging in such resistance have ended up being pulled into the sphere of irreligious atheism rather than into an alternative religious system. Atheism doesn't have to be irreligious nor does it have to be anti-religious, but cultural trends in the West have caused atheism, irreligion, and opposition to religion to be drawn together in such a way that there is now a high correlation among them.
As a consequence, atheism tends to be associated with being anti-religion rather than simply the absence of theism. This leads people to contrast atheism with religion rather than with theism, as they should. If atheism is treated as the opposite of and opposition to religion, then it will be natural to assume that theism is itself a religion - or at least some sort of anti-religious ideology, philosophy, world view, etc.