Philosophical taoism is agnostic, not atheistic. It's not based on the idea that God does not exist, but that we as human beings are not capable of grasping the Divine Mystery. Instead, we may align ourselves with the flow of existence as it is this flow of existence that is a reflection of the nature of the Divine Mystery. So that as we align with this way of being (the tao), we fulfill our place as reflections of the Divine Mystery from which we are manifest.
Taoism does not refer to deities not because it presumed them not to exist, but because it presumes itself incapable of understanding such things.
Hmmm - I think maybe we disagree as to the definition of "atheism". To me (as it is to all the other atheists on the forum, as far as I can tell) it is the
lack of a belief. Or, if you prefer, the
lack of a particular filter (belief in god) through which the world can be perceived. Nothing can be "based on" the simple and straightforward
lack of a belief in god, because there is
nothing there on which to base it. A belief in god is something that has been
added to the default position of no beliefs at all. So you can build on theism, because it's
there. Atheism is not.
Anyhoo, defining atheism as "a lack of belief in the existence of God or gods" (as atheists generally prefer) rather than "the doctrine or belief that there is no God" (as theists generally prefer), Taoism is
totally atheistic. There are no references to god/s at all, anywhere in the classic philosophical Taoist texts (TTC and Zhuang Zi, primarily). This demonstrates to my satisfaction that these Taoist philosophers
lacked a belief in god. Surely if they had perceived the world through the filter of theistic belief, it would have made an appearance somewhere in their musings. But it does not appear. Any philosophy where theism makes
no appearance is atheistic, IMO.
I should point out also that there is a giant pantheon of gods and demons in
religious taoism, headed up by the yellow emperor, presiding eternally in multiple heavens and hells.
But there is no "God". The religious variant of Taoism is polytheistic. The philosophical and empirical variants are atheistic.
When Lao Tzu says "the tao that can be named is not the Tao", he means this: The minute you attempt to define the Tao (i.e. by calling it "God"), you are no longer talking about the Tao.