ABANDONING Christianity in favour of other faiths continues to be increasingly fashionable in the west - and Buddhism remains one of the most seductive alternatives for those on a spiritual quest.

It may come as something of a surprise, then, to learn that one of the most eminent UK scholars of Buddhism has converted to Roman Catholicism.

Its doctrine is one that may be regarded as repressive and outdated, but after 20 years of teaching and practising Tibetan Buddhism, Professor Paul Williams has come to the conclusion that being a Catholic in the modern world is, at the very least, no less rational than being a Buddhist.

As a youngster, Williams, professor of Indian and Tibetan philosophy at Bristol University, immersed himself in the Anglican faith. He was baptised and confirmed a Christian, but, in common with many in the late 60s, "drifted away" after developing an interest in the exotic East and all things Indian. He studied philosophy and religion at Sussex University and went on to take a doctorate in Buddhist philosophy at Oxford.

"My academic work has always been bound up with my own search for answers and where we are heading, " says Williams. He admits he was among many who originally found Buddhism attractive because it appeared more rational than the alternatives.

Buddhism, he says, "seemed much more sensible - and much, much more exotic" - than a theistic religion (one that is based on a belief in gods or a God) like Christianity.

By the early 70s, Williams was already beginning to think himself a Buddhist. "I thought the notion of God was unlikely because there was so much evil in world.

Buddhism, historically, does not believe in God as creator. It sees this as a naive idea."

But today Williams believes his two decades spent as a Buddhist - after undergoing a ceremony to officially become one in 1978 - was always "deep down a sham".

This he attributes to the fact that he had no great conversion story to tell. "If asked, I responded with some embarrassment that I had spent so many years studying Buddhism that I had come to see the world as a Buddhist does. It just happened. Even though I went through the ceremony of 'taking refuge' in a Tibetan Buddhist context, I now think no real conversion took place."

Central to Williams' growing unease was his writing of a paper on reincarnation titled Altruism and Rebirth. It was then he began to truly appreciate the significance of the Buddhist claim that the reborn being cannot be said to be the same person as the one who died. "It really struck me that, of course, that is so right, " he says.

"So, although there is some karmic continuity, the chap I am now is finished when I die.

"I began to think that, if Buddhism was correct, then unless I attained enlightenment, or something like it, in this life - and Buddhists rarely do - I would have no hope. Neither would any of my friends or family."

It was through this realisation that Williams came to the conclusion that Buddhism is, in fact, hopeless. "Christianity at least offers some hope, " he says.

It also dawned on Williams that Buddhism - which downplays the role of faith because it is not based on the idea of a creator God - was not the purely abstract rational religion he had thought.

"In Buddhism, there are stories of people who live for 600 years, who f ly through the air as horses.

As a Buddhist, I also believed the Buddha was omniscient and I believed in the theory of karma [the belief that a person's state in this life is the result of physical and mental actions in past lives - a doctrine of moral cause and effect]. Now, I cannot quite see what grounds you could view that through apart from faith."

At that point, Williams decided to delve deeper into the notion of Christianity. "For all those years, I had not thought through the issue of God. Yet, my colleagues thought God could be rational and could be defended. So I started looking to see if I could answer the question of God using rational arguments."

Williams came to believe the existence of God was rationally defensible if viewed in terms of an "explanatory gap". "Buddhism has an 'explanatory gap' that, if it is to be filled, can only be filled with a necessary being, " he says. "This necessary being can also plausibly be taken as the God of theists. It is perfectly rational to accept the explanatory gap and not to seek to fill it. But, equally, it is rational to fill the gap with God.

"Since it seems to me that accepting the gap, and filling it with God, are equally rational, we cannot argue that one option is more rational than the other."

The next stage in Williams' conversion was to question the validity of the resurrection - when Jesus was said to have been killed on the cross and rose again. "The evidence for the resurrection being the most likely explanation of what happened at the first Easter is very strong, " says Williams.

"I believe Jesus genuinely died, the tomb was empty and he was subsequently, after his real death, physically alive, " says Williams. "I can't prove this happened, but it is a rational argument."

Williams sees Roman Catholicism as the "default position" of Christianity. "I was looking for a knock-down argument not to be a Catholic, " he says. "If I had severe reservations about the morals and sexual ethics in being a Catholic, then I think that would be getting to the stage where it was a knock-down argument."

Issues he looked at included accusations the Roman Catholic church became corrupt in the Middle Ages. He also took on the Church's stance on controversial topics such as contraception. "I asked myself if there was anything in these issues that made them knock-down arguments. The issue of abortion, for example, is not a problem for me. Even Buddhists don't believe in abortion. I didn't find any argument strong enough for me not to be a Catholic."

Williams believes he has shown in The Unexpected Way - a book he has written about his conversion - a rationale for following the "immensely sophisticated" spiritual path of Catholicism. He also defends that we cannot say, in advance, what God will do, and in some things we cannot know what God has done, short of him telling us in a revelation, such as through Scripture. "But God cannot be constrained, and that [Scripture] is only one form of revelation, " says Williams. "Another major form of revelation is through the tradition and teaching authority of an authoritative Church."

Part of the problem for westerners, believes Williams, is that their knowledge of Christianity's thought and practical experience is elementary. "It is what we learned at school. It is no more sophisticated than that of an 11year-old, " he says. "When we come to Buddhism as adults, we immediately start studying it at a level that traditionally would have been the preserve of an elite of highly talented practitioners."

"Christians are not as stupid or as gullible as some people might think. It saddens me to find those who think Christianity spiritually and doctrinally naive and impoverished compared with Buddhism.

"Of course becoming a Christian is a gamble. Eventually we do have to stop thinking about the arguments and make a choice, and my choice has been rationally defensible. I hope my story will chime with other people."

The Unexpected Way by Paul Williams (Continuum books/T&T Clark International, [pounds]16.99)