Bertrand Russell did not consider himself an expert on ethics and religion, and it is true that his writing on these subjects lacks the originality and sophistication of his philosophical work on mathematics. His criticisms of religion are often similar – in essence if not in tone – to opinions voiced by contemporary atheists: he argued that religious beliefs cause wars and persecution, are moralistic and oppressive, and foster fear. However, it is precisely for this reason that it is worth looking again at Russell's rejection of Christianity. Anyone concerned with defending religion against its typical modern detractors must recognise Russell as a worthy opponent, for he was an intelligent, principled and humane man of the world who undoubtedly led a meaningful life.

Next week we will begin to look closely at Russell's arguments against Christianity. First, though, let's consider how his general attitude and approach to religion shaped his critique of religious beliefs. It is telling, for example, that Russell thought that religious questions did not really belong to the discipline of philosophy. This rather narrow view of philosophy predisposed him to scepticism about subjects that involve ambiguity, interpretation, and perhaps a personal, experiential kind of insight. Ethics, of course, is one such subject, and religion even more so. Like earlier rationalist thinkers such as Descartes and Spinoza, Russell had an exacting standard for what qualified as "knowledge", and argued that if philosophy is the search for truth then it should concern itself only with the kind of certainty associated with basic mathematical intuitions such as "2 + 2 = 4".

It is also interesting to compare Russell's dismissive attitude to religion with his great faith in science. When Nietzsche wrote of the death of God, he suggested that belief in scientific progress was the last remaining article of faith. Nietzsche was pointing out that although science makes claims to knowledge, these claims are as deluded as those of religious dogmatists. The view that he was criticising is too crude to attribute to Russell, who acknowledged what we customarily call "knowledge" occupies a broad spectrum of degrees of uncertainty, and that very little – if anything – is absolutely certain. Nevertheless, it is worth bearing in mind Nietzsche's remarks about the "piety" underlying modern science when we consider Russell's almost utopian vision of scientific progress.

Russell's support for eugenics in his eccentric and provocative book Marriage and Morals (1929) is one of the more controversial examples of his view that scientific developments could, and should, contribute to social reform. But this view itself has become a tenet of secular orthodoxy. It is articulated with characteristic eloquence in Russell's essay How I Came By My Creed, which was published in the same year as Marriage and Morals. Here Russell celebrates our increasing mastery of nature, and argues that modern science both overcomes religion and replaces it as a method for humanity's self-improvement: "In this world we can now begin a little to understand things, and a little to master them by help of science, which has forced its way step by step against the Christian religion … Science can help us to get over this craven fear in which mankind has lived for so many generations. Science can teach us, and I think our own hearts can teach us, no longer to look around for imaginary supports, no longer to invent allies in the sky, but rather to look to our own efforts here below to make this world a better place to live in, instead of the sort of place that the churches in all these centuries have made it."

This passage is typical of Russell's popular writing on religion, and it is not surprising that contemporary atheists have adopted him as their champion. But his autobiography occasionally reveals a more complex and ambivalent relationship to religion. In particular, he relates an episode in 1901 when he witnessed the wife of his Cambridge colleague Alfred Whitehead suffer intense pain due to heart problems, causing Russell to have what can only be described as a spiritual insight. "The ground seemed to give way beneath me and I found myself in quite another region," he writes. "Within five minutes I went through such reflections as the following: the loneliness of the human soul is unendurable; nothing can penetrate it except the highest intensity of the sort of love that religious teachers have preached; whatever does not spring from this motive is harmful, or at best useless; it follows that war is wrong, that a public school education is abominable, that the use of force is to be deprecated, and that in human relations one should penetrate to the core of loneliness in each person and speak to that."

Such was the power of this experience that it made him "a completely different person". Even though Russell's "mystic insight" later faded in the face of an older "habit of analysis", its effects, he wrote, "remained always with me, causing my attitude during the first war, my interest in children, my indifference to minor misfortunes, and a certain emotional tone in all my human relations".

What caused the disparity between Russell's "official" view of religion and his personal experience? Why was he unwilling to bring this experience to bear on his critique of religion? The answer seems to lie in his deep methodological commitment to both rationalism and scientific empiricism: Russell tended to treat "religion" as either a body of doctrines to be intellectually analysed, or as a phenomenon to be observed objectively from the outside. In the first case, Russell found flawed arguments; in the second, flawed institutions perpetrating violence and oppression. His own spiritual insights belonged to a different order – and although they changed his life deeply, they were not allowed to change his philosophical position. This helps to explain why, while history proves that both religion and science can be forces for good and for ill, Russell was inclined to focus on the benefits of science and on the dark side of religion.