The whole pre-trib secret rapture idea started with a prophecy from a 15 year old girl in Scotland who was dabbling in spiritism and seances.
http://en.wikipedia.org/wiki/Margaret_McDonald
In fact the church she attended was itself into all sorts of dark magick. It actually studied the Kaballah.
http://www.truth-not-tradition.com/content/singles/Church%20History%20Pre-Trib%20And%20The%20Occult_1099.html
John Nelson Darby later latched onto the pred-trib idea because it gave him a way to pretend that certain prophecies about "God's elect" "Christ's elect" and the kingdom of Christ on earth would be fulfilled not by the church, but by the Jews.
There really is no valid scriptural support for the pre-trib doctrine, but Dispensationalists can't let go of it, because if they do, their whole house of cards comes tumbling down.
