If you read Revelation, it seems that during the times of the anti-christ this world will be very corrupt and wicked. It is also apparent that while there will still be religious faiths in the world, people will generally not be believers. So will there be a big world religion in the future?
To be honest, I think there will always be some diversity in terms of religious beliefs.
But if there was going to be a major religion, it would probably be disbelief/atheism. I say this because many people are rejecting God due to a bigger belief in science alone.
But personally, I hold on to God and also appreciate science.