As a kind of curiosity, I think that there are monist non-materialist atheists. In fact, I think Bertrand Russell for a period was like this, and was described as a neutral monist, meaning that there is one substance in reality, which can appear as mind or matter. I don't know if it has carried on as a philosophical standpoint.
And there have been idealist monists, or mentalist monists, who argue that there is only consciousness in reality. I suppose it became very unfashionable in the 20th century, but for a while was common, but usually such people would be also theists.
And in some Eastern thought there are ideas like this, that there is only mind, well, Mind. But some of these people are not theists.
Another version of this is panpsychism, again, meaning that there is only mind. Surprisingly, it has come back into fashion in philosophy, e.g. Galen Strawson.