Bertrand Russell's dissent from what was still, in his time, conventional Christian belief can be explained in part by his background and early influences. His grandmother brought him up as a Unitarian, which meant that "eternal punishment and the literal truth of the Bible were not inculcated", as he puts it in his autobiography. Like his free-thinking parents, Russell was impressed by John Stuart Mill's utilitarian philosophy, which he first encountered as a teenager. But his critique of Christianity was also due to the fierce intellectual integrity with which he confronted every issue he found worthy of reflection. At the age of 14 Russell began to question the tenets of Christian faith – including free will, personal immortality, and the existence of God – and by the age of 18 he had rejected them all.

However, the 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." Theoretically, agnosticism is very different from atheism, for atheists and theists share the conviction that knowledge about such matters is attainable – and, indeed, that they have attained it while their opponents have failed to do so. However, from a practical point of view Russell admits that agnosticism can come very close to atheism, for many agnostics claim that the existence of God is so improbable that it is not worth serious consideration.

In his 1927 lecture Why I Am Not A Christian Russell describes God's existence as "a large and serious question", and he rejects some of the classical theistic arguments – the first cause argument, the design argument and the moral argument. (He does not here consider the ontological argument, but in his famous 1948 radio debate with the Jesuit philosopher Frederick Copleston he argues that the concept of a necessarily existent being, which is central to the ontological argument, is nonsensical.)

The lecture also criticises the character of Jesus presented in the gospel narratives. In particular, Russell rejects the idea of hell: "It is a doctrine that put cruelty into the world and gave the world generations of cruel torture; and the Christ of the Gospels, if you could take him as his chroniclers represent him, would certainly have to be considered partly responsible for that." On the other hand, he admires certain principles of Jesus's teaching, such as refusing to judge others and being generous to those in need, although he finds them "difficult to live up to". The idea of hell is certainly challenging for both believers and non-believers, but it is difficult to engage with Russell's critique when he does not himself engage with centuries of theological reflection and debate on this issue. For example, he does not consider the Catholic teaching that hell is a separation from God that is not inflicted as a punishment, but freely chosen by human beings.

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.

More than this, though, Russell's agnosticism itself has a spiritual dimension. Suspending judgment about metaphysical questions is a sceptical intellectual practice, but a more radical suspension of judgment belongs to what Russell describes as "contemplative worship" in his 1912 essay The Essence of Religion. Here he attempts to sketch a kind of spirituality that is based not on belief in God, but on "the contemplative vision, which finds mystery and joy in all that exists, and brings with it love to all that has life". Russell finds three elements within Christianity that he wants to preserve: "worship, acquiescence, and love". The "impartial" worship he envisages "has been thought, wrongly, to require belief in God, since it has been thought to involve the judgment that whatever exists is good. In fact, however, it involves no judgment whatever; hence it cannot be intellectually mistaken, and cannot be in any way dependent on dogma." In other words, genuine contemplation is by nature non-dogmatic, since it departs from our ordinary mode of judgmental thinking. The ethical counterpart of this contemplative attitude, of course, is the refusal to judge others that Russell so admired in Christian teaching.