The most powerful aspect of Bertrand Russell's critique of religious belief is his claim that religion is based on fear, and that fear breeds cruelty. His philosophical arguments against the existence of God may not touch the lives of many ordinary people, but his more psychological point about fear has to be taken seriously by all of us. In his 1927 lecture "Why I am not a Christian" – delivered to the south London branch of the National Secular Society – Russell expressed his point with characteristic clarity: "Religion is based primarily and mainly upon fear. It is partly the terror of the unknown and partly the wish to feel that you have a kind of elder brother who will stand by you in all your troubles and disputes. Fear is the basis of the whole thing – fear of the mysterious, fear of defeat, fear of death. Fear is the parent of cruelty, and therefore it is no wonder if cruelty and religion have gone hand in hand. It is because fear is at the basis of those two things." No doubt he was preaching to the converted on this occasion.

There are actually two elements to Russell's diagnosis of religion here. The first is that religious belief is a symptom of fear: aware that our lives are precarious and vulnerable, we seek the protection of a powerful deity, to comfort ourselves with an illusion of safety. The second is that fear is a symptom of religion: in particular, doctrines of punishment in both this life and the next cause ignorant believers to live in fear unnecessarily. There is little doubt that this analysis has some truth on both points; perhaps it explains quite accurately the causes and effects of religious belief in a significant number of cases. But do such cases represent religion itself, or are they a distortion of it?

We will focus here on Christianity, since this is the tradition that Russell was mainly concerned with. While Russell argues as if his rejection of fearful belief and fear-inducing dogma comes from an atheistic perspective, the Christian tradition itself contains a vigorous critique of fear. The First Letter of John, for example, puts forth the basic tenet that "Whoever does not love does not know God, for God is love", and suggests that fear and love are incompatible with one another: "There is no fear in love, but perfect love casts out fear; for fear has to do with punishment, and whoever fears has not reached perfection in love." In fact, Russell echoes this sentiment in a 1912 essay on "The Essence of Religion, where he writes that "fear tends more and more to be banished by love, and in all the best worship fear is wholly absent." But he did not need to appeal to any biblical text in arguing that "fear is the parent of cruelty", because it is a basic psychological fact that love is inhibited and distorted by fear.

In the 17th century, Spinoza – whom Russell described as "the noblest and the most lovable of the great philosophers" – invoked the First Letter of John to attack the persecution of non-conformists by the Dutch Reformed church. The violent dogmatism witnessed by Spinoza is exactly the sort of thing emphasised by modern atheists who claim, like Russell, that religion is a harmful force in the world. But Spinoza attacked "superstitious" forms of religious belief, which are characterised by fear, as a dangerous perversion of a purer Christian teaching found in the New Testament. Prefacing his Theological-Political Treatise with a verse from the First Letter of John, Spinoza implied that the church was failing by precisely those Christian ethical standards which it claimed as its own.

Another example of a Christian critique of fear can be found in Kierkegaard's analysis of the theological concept of sin. Traditionally, pride has been identified as the fundamental form of sinfulness, but Kierkegaard argued that human psychology is darkened by an inseparable combination of pride and fear, which both get in the way of love. This means that the Christian ideal of love requires us to battle against both pride and fear, to combine humility with courage. According to Kierkegaardian theology, fearful religion is sinful religion.

These two brief examples suggest that the Christian tradition has the resources not only to recognise the dangerous consequences of fear, but to scrutinise them closely and provide a spiritual response to them. However, this is not the sort of perspective that Russell was prepared to explore in his philosophical work. He was certainly unwilling to invoke the Christian doctrine of original sin – presumably because it was closely associated with the Victorian moralism that, to Russell's disgust, lingered long into the 20th century.

But his atheist disciples may be surprised to discover that privately Russell found some meaning in the concept of sin. In his autobiography he describes a visit in 1952 to a small Greek church, where he became aware within himself of "a sense of sin" which, to his astonishment, "powerfully affected" him in his feelings, though not in his beliefs. If Russell had followed Kierkegaard in paying more heed to such "feelings", he might have come closer to understanding that fear is a religious problem, and not just a problem with religion.