The False Prophet is a literary term (from Revelation) for a character in DEXTER. The False Prophet is depicted as an agent of "The Beast" and predicted to be cast within The Lake of Fire along with its master.
According to Travis Marshall, Dexter Morgan was The False Prophet that tried to lead him "astray" from the Path of God. At first, Dexter believed that Travis was innocent while Professor Gellar was the true Doomsday Killer. In reality, Gellar was dead from the start and Travis acted alone while believing in an illusion that he considered to be Professor Gellar.
The term "False Prophet" was used by Professor Gellar which, in turn, led to Travis accepting the concept. Travis began to express his darkness in the form of Gellar. After Travis realized the truth that he had killed Gellar, he stopped seeing Gellar's illusion. He then consistently named Dexter as The False Prophet.