Scenario Three

Churches outside mainstream Christianity are growing the fastest. The greatest growth in the future is expected from such marginal church groups as the Mormons and the Amish.

The growth, however, will not go without criticism. These churches will be, and some already are, considered cults or heretics by the orthodox, Kelly points out.

Nevertheless, Kelly says "an entirely safe bet would be that the largest denomination 1,000 years from now is one that does not exist at the moment".

Currently, the largest church in the United States is Lakewood Church, a non-denominational church of around 40,000 weekly attendants in Houston. It was founded in 1959.

Scenario Four

An overwhelming majority of the challenges - such as abortion, stem cell therapies and pornography - Christianity will be facing in the next millennium will be driven by new technologies. Kelly points out that today's challenges are tame compared to the ones coming.

And as Christians have already been witnessing, the next generations of Christians will speak the Facebook and YouTube language as easily as Americans speak English today.

"The long-term trend is more technology in the Christian culture; what is missing, and what may take several generations to supply, is an understanding of the spiritual meaning of technology," Kelly writes.

Scenario Five

As culture continues to move towards a future of questioning and doubt, Christianity has to "develop a cultural practice of positive questioning, of active holy doubt, and a clear articulation of what is eternal and what is in flux," he notes.

That practice is likely to be constructed not by theologians, Kelly says, but by members of the worldwide church in a distributed social media context - "the wiki-church".

And that includes Muslims.

The Christian community is shrinking in Europe while the Islamic community continues to grow. And while Islam has turned radical and militant in other parts of the world, Kelly points to the millions of non-militant moderate Muslim communities.

"On many social issues moderate Islam and conservative Christianity agree," he contends. "They are both people of the book. They both honor many of the same prophets. They agree on many religious issues like prayer, sexuality, sin, and family.

"It is not impossible to imagine Muslims and Christians becoming allies in the inevitable culture wars of the future. It is no more impossible than imagining Christians and Jews would be allies a thousand years ago."

Kelly predicts 100 years from now, a conservative Christian-Islam alliance might be a serious global political force.

While none of the five scenarios may happen, they are presented in order to gain a firm grasp of the present trends, Kelly says.

"Sometimes it takes an exercise of extrapolating to a thousand years from now to see what is happening tomorrow. Only by extending a trend can we see if it might endure, or survive in the face of other trends, or if it might provoke an awareness of a trend we could not see before."