When I left the Catholic Church, the traditional idea of a personal supreme deity became just absurd to me. I was a strict scientific materialist for about 2 years, and found it almost completely satisfying, but the sense that there was something more never really left me (the sense of the numinous, I think it's sometimes called), even though I tried. I eventually accepted that I was too spiritually/mystically minded to remain a materialist.
I find that Taoism most closely expresses my understanding nowadays. Some people might call the Tao a "god", but it's totally impersonal, more of a principle than an entity, and nothing at all like the Triune God of my youth.
But along with that go some ideas that a strict materialist would probably reject, such as the notion of chi or its use in reiki energy healing. But my life goes smoother when I try to follow the principles of the Tao, and I have personally experienced the effects of chi, so it works for me.
I call it atheist in the original sense, a-theist, not against god, but simply without god. I also consider myself to be an existentialist: there is no purpose to life except what we ourselves make of it. I find this idea very liberating, and I don't understand why some people think it's bleak.