I recently had a conversation on a mailing list about the religions that would arise after the oil ran out (or, more precisely, after the financial and energy cost of accessing the remaining oil became prohibitive). Among the participants was John Michael Greer, whose Archdruid Report is one of the finest online resources on peak oil and the post-technological world. John has written a number of books on the subject, including The Long Descent: A User's Guide to the End of the Industrial Age and The Ecotechnic Future: Envisioning a Post-Peak World. If you're looking for an expert's opinion, drop by his blog or pick up some of his books: what you are going to get here are the musings of an interested amateur.
In our discussion, JMG rightly pointed out that the new religions which arose in the wake of Rome's collapse were dark horses: within a century or so Christianity went from a nutty slave-cult to the Empire's official religion, and a couple centuries after that the chaos in the former Diocese of Oriens and Persia's Sassanid Empire helped an obscure desert religious leader become the Seal of the Prophets. But while we can't predict exactly what religious movement will become the Next Big Thing, we can assume the future will in many ways be a reflection of the past.
Christianity became a powerful force because it represented the past. It's no coincidence that the Orthodox Church looked to Constantinople and the Eastern Roman Empire while Western Christiandom looked to Rome. In both cases the Church presented itself as the survival not just of the Kingdom of Christ but of the Roman Empire. It didn't just stand for salvation in heaven but for the glories of the past. The surviving literature of the Classical Era was in significant part preserved by the monasteries and church universities.
The feudal system of the Dark Ages was a low-budget form of the Pax Romana. In exchange for becoming landless serfs instead of freeman farmers, the peasants received protection from bandits, robbers and rampaging hordes. The scarce resources were largely concentrated in the hands of feudal lords and clerical leaders (who were generally connected through blood ties or political alliances). The system was not nearly so efficient as Roman rule: crime rates soared during this era and invasions by one barbarian tribe or another were fairly commonplace. But its existence was justified as a survival of "Christiandom" -- which was essentially synonymous with "the Roman Empire." Charlemagne and the rulers of Byzantium explicitly made this connection, even as their "empires" crumbled around them.
By contrast, Islam represented something New and Different. Muhammad (SAW) drew from a number of religions which were found in the Arabian peninsula, but his Qur'an was a New Dispensation. Instead of hearkening back to the halcyon days of the Roman Empire, it promised a glorious new future when the whole world would say "there is no God but Allah and Muhammad is his Prophet." This vision of earthly and heavenly paradise captured the hearts and minds of people on the empire's eastern frontier and became a major threat to the One Holy Catholic and Apostolic Church.
Paradoxically, these "barbarian hordes" did a better job of preserving Greek and Roman knowledge than the old-timers. Because they were part of a new movement, they had less pressure to conform their scholarship to the "accepted way" of doing things (read: a badly-remembered mixture of tradition and superstition. Imagine the Texas Association of School Boards on steroids and you'll have some idea). And because Islam was a growing faith while Christiandom was beleagured on all sides, there was far more tolerance for unorthodox ideas. Diversity of ideas is a luxury which you can afford when you're firmly in charge: it becomes far less palatable when your position at the top of the heap is threatened.
(I admit that the above is an oversimplification of a very complex social order, but it is not an entirely unfair one).
I suspect the religions of the Long Descent will follow a similar pattern. On one hand you will see one or more religions which look back to the Old Days. They will use the remaining resources to protect their Have-Very-Littles from the Have-Nots, promising them order amidst the chaos. Not coincidentally, they will also use these resources to protect themselves from the Have-Very-Littles. I would expect them to keep some form of the Internet (which will be used primarily for communications and military purposes rather than as a repository for streaming video porn) and many forms of mechanized military weaponry. (Such gas and oil as is available will largely be used for those purposes, in the name of "preserving our Ancient Culture").
This religion will be the dominant faith of the cities and the former First World. It may call itself "Christianity" or "Islam" or it may claim those faiths as its primary influence. It will provide what education is available to those can afford it. Such education will be along rigid dogmatic lines with little tolerance for those who deviate from the One True Way. It will regularly engage in purges against "heretics" and "infidels" who question its preachings or its hold on power. You can expect regular replays of the Cathar-b-ques and Waldensian massacres of the Middle Ages.
Their primary competitors will be new religions led by prophets. These wild-eyed visionaries will promise a brave new world arising out of the ashes of the decadent Old Empire. They are likely to arise in the "Developing World," (a problematic term, I admit, but I don't have a better one handy) and may well be influenced by that old atheist prophet Karl Marx. These areas have seen the benefits of "progress" from afar and may have more use for that notion than a religion which is primarily concerned with maintaining order and stability. These new faiths will become a major threat to an establishment which is already faced with banditry and anarchy in its far-flung reaches (which, as gas becomes more scarce, could mean its exurbs). And, in time, they could serve to unify those bandits and barbarians into a powerful new force.
I'd be interested in hearing what others have to say about my thoughts. As I said, I'm a history buff rather than a trained historian. But this seems to me a plausible vision of the future and its major religious movements.