I recently remarked about my frustration with this frequent claim from theists when reviewing True Reason. Co-author and editor Tom Gilson stopped by to offer his comment on the matter, saying:
[If] you don't know any popular atheists who presuppose metaphysical naturalism, all I can say is wow, how does that sand feel in your ears, nose, and eyes, and how do breathe with your head in there?As a brief reply, I offered two examples of popular atheists saying the contrary: Richard Dawkins' assertion in The God Delusion that God's existence is improbable, not impossible; and Sam Harris saying, in one of his debates, that "science is not, in principle, committed to the idea that there is no afterlife, or that the mind is identical to the brain, or that materialism is true." 
I could probably keep searching around the writings of popular atheists and find similar statements, but I don't think that would be germane to my point. After all, this is my blog, and my views may or may not represent the views of others. Maybe there are dogmatic materialist atheists out there, but I don't know of any. But even if there are, that's not the position that I take. The important question, as I see, it whether atheism necessarily entails materialism, and I think it's abundantly clear that it does not.
Here's why. When we are given a proposition x, there are not only two options: x is true, or x is false; x may also be indeterminate. We may reject ideas as indeterminate because they are poorly structured (containing logical inconsistencies), or because they lack evidence for their claims.
Consider for example an article I read recently in Scientific American on the multiverse. The author, physicist George Ellis, argued that the litany of multiverse theories are poorly formed, can be molded to 'explain' virtually anything (there's an old saying that a theory which proves everything proves nothing), and may in principle be unfalsifiable. This is not sufficient evidence to deny that a multiverse exists, as Ellis acknowledges in the article; however, it's grounds to refrain from affirming that it does, because it is indeterminate.
This is a critical distinction that, to my great frustration (and that of many other atheists I've encountered), theists just seem to have a hard time with. We view the existence of God as indeterminate. We view the evidence as insufficient and/or the basic concept of 'gods' or 'God' to be logically incoherent (see ignosticism). This is sufficient for us to reject holding a positive belief in a God or gods – hence atheism. This does not imply that God's existence is impossible or that materialism must be true – only that supernatural concepts are lacking coherency and/or supporting evidence.
Sometimes, theists attempt to corner the non-believer into affirming materialism by cajoling them into answering 'big questions', like "If there's no God, how do you explain the existence of the universe?" To quote the physicist Sean Carroll,
[Some] things happen for “reasons,” and some don’t, and you don’t get to demand that this or that thing must have a reason. Some things just are. Claims to the contrary are merely assertions, and we are as free to ignore them as you are to assert them.In other words, we don't have a burden to answer the question simply because the theist has demanded that the universe must have an explanation for its existence; it's entirely possible that, per Stephen Hawking's No Boundary Proposal for example, the universe simply is. Again, we do not have to assume or affirm that this is the case – its plausibility, coupled with the fact of all we simply do not know about the universe (such as what lies beyond the Planck Epoch in the realm of quantum gravity) is sufficient to show that the theist's demand for an explanation is rooted in a baseless assumption.
I feel that this distinction is so critical to properly understanding atheism that it really cannot be understated. I'm going to finish with a great video from Youtuber QualiaSoup that he put up today on "substance dualism" – the idea that the mind and the brain are distinct entities, which is quite common if not necessary in many branches of theology. Note that he does not reject the idea by saying that materialism must be true, or that he can disprove the existence of "non-physical" things; rather, he rejects dualism because it is conceptually incoherent and lacking evidence. He could rightly be called an "adualist".