""It is undesirable to believe a proposition when there is no ground whatsoever for supposing it is true." — Bertrand Russell"When it came to the grounding of the "logic" of Atheism, Russell was attached to fallacies such as the one above. Atheists love it because they think it supports their worldview without any thought on their own part, and the simple Appeal to Authority. Russell was attempting two things, it appears. First he was trying to state an old old principle of logic, in a Russell sort of format. Second, it is likely that he made this statement regarding the use of Materialism to attack religious conceptions of deity.
But it is only half true, and therefore false; here's why. It is also perverse not to accept the truth of a proposition which is deduced using valid structure, premises that are true and grounded in the First Principles, and which passes Reductio Ad Absurdum. This is the half of rationality which Atheists, including Russell, would like to ignore, because it applies directly to them and their worldview.
Let's see what Russell is actually saying, if it is applied to Atheism.
When Atheists claim to have no theist beliefs, the ground for that claim is missing because the implications are that, (a) either they are ignorant of theist claims, which is not the case, or (b)they have rejected theist claims without any reasons or reasoning, much less empirical data to the contrary. So Atheism, taken as "no theist beliefs" has no ground for supposing that it is true". Further, it is false because they do have positive beliefs regarding the existence of a creating deity for the universe, positive beliefs regarding the truth value of major religions, and positive beliefs regarding the nature of existence which eliminate all intelligent input in favor of determinism.
It is not possible to call oneself an Atheist without having a concept of theism; claiming otherwise is either disingenuous or massively ignorant. Yet that is the claim: Atheists have no concept of theism, and therefore are not subject to any requirement to support any rejection of theism. This is logically impossible, so is either indicative of widespread irrationality amongst Atheists, or the propensity to lie regarding their beliefs in order to protect themselves from intellectual responsibility, or - more likely - both.
But even more to the issue is the fact that there is no case to be made for Atheism, if it is not rejection. There is no positive case to be made that there is not and cannot be a creating entity for the universe, neither empirically nor deductively. So everything about Atheism is subject to the first claim by Russell, above: it is undesirable to believe a proposition [such as Atheism] when there is no ground whatsoever for supposing it is true.
As is their wont, the Atheists stumble into logical pits of their own construction, then deny that there is a pit at all by claiming to have no opinion about it.