"Are you a Christian?" The question isn't often asked today in a neutral tone. If you mutter a vague "yes", there is always a supplementary seeking a more precise formulation. "Do you believe that Jesus was the Son of God?" "In the resurrection?" And so on.

Elaine Pagels argues that this need to define belief is not the product of a scientific and sceptical age, but rather of a long-standing drive within the churches to confine religious sentiment in a code of belief. Hence the Penny Catechism of my Catholic schooldays and the Universal Catechism today: attempts to sum up what many hold to be an invisible, intangible spiritual dimension in a list of rules about diet, devotions and sex. Hence, too, the deep suspicions aroused in ecclesiastical circles by mystics down the ages, inspirational figures like Teresa of Avila or John of the Cross. Their visions of the divine could never fit easily into any right-or-wrong formulation of orthodoxy.

This tension between authority and revelation is at the heart of Beyond Belief. Pagels, professor of religion at Princeton, has returned to territory she first covered in her 1979 classic on the Gnostic gospels: the first- and second-century writings about Jesus excluded from the Christian canon, but which turned up in 1945 at Nag Hammadi in Upper Egypt, buried in a large jar. In particular, Pagels is fascinated by a poetic account of Jesus's life known as the Gospel of Thomas.

There are close parallels, she shows, between the Gospels of Thomas and John: the youngest of the four in the New Testament and itself straining for spiritual truth about Jesus, in contrast to Matthew, Mark and Luke. Yet where Thomas's Jesus openly celebrates the divine "light" within each human being - and so allows everyone the potential for a mystical bond with God - John's Jesus insists we can only experience God through him; and so, it was argued by early church fathers, only through the structures that continue his work, namely the church.

Pagels paints Thomas and John as rival statements from competing factions within the youthful church. John's triumphed and put Christianity on a course that favoured an enforceable canon of truth over revelation, religious box-ticking over religious experience.

She illustrates the consequences with an account of her own visit to church when her two-year-old son was terminally ill. As the congregation chanted the liturgy, it sounded "like barely intelligible signals from the surface, heard at the bottom of the sea". But also there was a realisation that "here was a place to weep: to deal with what we cannot control or imagine". She was attracted and repelled simultaneously.

This is writing about religion of the first order: enlightening, intelligent, inclusive and humane. It even provides a few good lines to silence those persistent questioners who demand the low-down on your faith.

The reviewer's 'Heaven: a traveller's guide' is published by HarperCollins