Despite the philosopher’s reputation as a staunch unbeliever, Clare Carlisle argues that he walked the line between atheism and agnosticism:

[T]he same intellectual integrity that made Russell unable to accept religious beliefs also prevented him from embracing atheism. Rather like the 18th-century Scottish philosopher David Hume, Russell maintained a sceptical attitude to metaphysical questions. He explains this position very clearly in a 1953 essay on his agnosticism, where he states that, “it is impossible, or at least impossible at the present time, to know the truth in matters such as God and the future life with which Christianity and other religions are concerned.” …

Although Russell often seems in his writings to be drawn towards a quasi-atheist position, his own agnosticism is reinforced by his recognition that the word “religion” does not have a very definite meaning. “If it means a system of dogma regarded as unquestionably true,” he writes, “it is incompatible with the scientific spirit, which refuses to accept matters of fact without evidence, and also holds that complete certainty is hardly ever attainable.” The agnosticism article was published at a time when critics of religion were often assumed to be communists; Russell counters this suggestion by pointing out that the kind of communism advocated by the Soviet government fits his definition of dogmatic religion, and that therefore “every genuine agnostic must be opposed to it”. It is clear that a passionate aversion to dogmatism runs through both his critique of religious oppression and moralism, and his more positive doctrine of philosophical agnosticism. Russell sometimes seems to be moving towards the view that how ones believes, and not just what one believes, is ethically significant – a view that will be embraced by any reflective religious person.

Previous Dish on Russell here, here, and here.