If you look at Catholicism, the people who practice it worship Mary and the Pope and participate in non-Biblical rituals. Do you suppose there will be a large number of them left after the Rapture? That would seem to be a large group of people to possibly be the one world religious system.
The totals vary by survey, but the number of Catholics in the world was estimated at 1.18 billion as of 2010, compared to 1.57 billion Moslems. I have a hunch there are a lot more born again Catholics than we might suspect, although no one but the Lord knows for sure. But no matter what the number, the Catholic church will probably be left in disarray after the rapture making Islam the largest and fastest growing religion in what’s left of the world by an even larger margin than before. I wouldn’t be surprised if many of those Catholics left behind look to Islam, since the two religions have much in common.