If you look at present trends and project a few hundred years into the future, you will see that Christianity will be reduced to a small cult religion, possibly underground as it was in the early days of Rome.
I can see three different scenarios:
1) After the fall of the US Empire, Islam, or versions of it, will dominate within totalitarian states and possibly all other religions will probably be banned or severely restricted.
2) Society at large would be divided into two distinct groups of rich and poor with many sublevels within the two communities. The richer classes will be mainly atheistic but oddly pious, while the others will develop myriad cults and offshoots of existing religions, which some of the richer classes will aspire to, infiltrate and take over.
3) After a nuclear holocaust that wipes out most of mankind and reduces cities to wastelands, the children born in an old school house located next to a shopping mall that both miraculously survived, having lost their parents young and having no knowledge of the written or spoken word, beget more children as they grow older, some of which survive and so forth until, after a few generations, the growing and ignorant population begins to revere the blackboard salvaged from the place that spawned their ancestors. Intrigued by the by now faded chalk marks scrawled across and down it depicting words they could never understand, a message that will eventually be interpreted by future intellectuals as meaning any number of things they choose it to, turning the board into a holy shrine, a symbol of the god that spawned them in that old school house and starting a new religion.
And as they begin to mutate from radiation poisoning, they will revere the old photos and mannequins of beautiful people that had been salvaged from the shopping mall and those images will become their gods and goddesses.