Heather Bellamy spoke with eminent historian Geoffrey Blainey.

Heather: Some of the key names in the history of Christianity would be Constantine and Luther. Who were they and how did Christianity change because of them?

Geoffrey: Yes, both were very important weren't they? Constantine was the emperor and the head of the Roman Empire in the three-hundreds. He put the State behind Christianity, so instead of Christianity being persecuted, Christianity was the favoured one. That brought disadvantages as well as advantages, but it was a great landmark in the rise of Christianity.

Luther, more than a thousand years after Constantine, was very important. He's the one who really puts an emphasis on the Bible. He said you had to read the Bible and you had to gain faith in God rather than do good works and in that way buy your way into his favour, or buy your way into Heaven.

In some ways Luther is a bit like the way people are interpreting Brexit or similar movements at the moment. Luther was the first rebellion against the elite. He believed that people with money got a better deal from Christianity than those who had no money. People with money could buy their way into Heaven whereas those without money were just in the long queue.

Heather: Why do you think Christianity has declined in the West in recent decades?

Geoffrey: I'm inclined to think the main reason is material success. People live longer and they have more comforts. They have less fear of starvation, but at the same time I think one of the key messages of the Bible is that mankind can't live by bread alone. You need more to life than material goods. We happen to belong to the first couple of generations in the history of the world where we can live on bread alone, or we think we can live on bread alone.

I don't think that's going to be a permanent state of mind and I see a period when Christianity will revive again, when we see that all the material comforts and all the plenty that we have surrounding us at the moment, or in most parts of the world, they're just not enough for a satisfying and a stimulating life.

Heather: Where would you say the heartland of Christianity is now?

Geoffrey: In my book is a map of the world showing the top 10 countries, if the statistics are reliable, and there's only one European country in the top 10 and that's Italy and it's near the bottom of the 10.

I don't mean that Christianity isn't widespread in Europe and if you took the European Union as one nation, then it would be on top of the world, but of course as we know it's not really a nation.

The two top nations in the strength of Christianity I would think are the United States first and Brazil second. Then Africa has lower down the list, three nations that are in the top 10.

So the heartland of Christianity is moving away from Europe, but that's partly because the population of Europe is static. Whereas the population of Africa and parts of Asia is soaring and the Christians are increasing with that soaring population.

Heather: What is your opinion of Christianity?

Geoffrey: After looking at Christianity in the space of 20 centuries, my opinion of it, is it's an influence for good. I know at times Christianity, or Christians, have been an influence for bad, but my impression is favourable.

I'll just read you the last two sentences of the book, "The debate about Christ's message and influence will continue long after we are all dead and the twenty first century is lost behind passing clouds. The fascination with Christ will persist and many will still see him as triumphant." I add a little earlier, "It's remarkable that a man who lived 2,000 years ago, who held no public office and owned no wealth and who travelled no more than a few days walk from his birthplace should have exerted such influence on the world for so long." That's my conclusion.

Heather: How can people get copies of your book?

Geoffrey: It's a Penguin book in Australia and New Zealand and in various other countries, but in England it's been taken up by the very old publishing company SPCK. You can buy it in bookshops in the British Isles. It has other publishers in other parts of the world and is already coming out in various editions.