The evangelist is a homeless man who preaches on the streets of Philadelphia. As you can probably guess, his set list includes all the hits from the Book of Revelations and not much else. (Why is it that street evangelists love Revelations so much? You'd think they would throw in some Ecclesiastes, if only to mix things up.)
But this guy's jam is Revelations, specifically 15:7, which goes a little something like this:
EVANGELIST: One of the four beasts gave unto the seven angels seven golden vials full of the wrath of God, who liveth forever and ever.
You may have noticed that this is the same verse quoted by Dr. Railly during her seminar, and it touches upon many of the story elements of 12 Monkeys—namely disease, the end of the world, and foretelling future events.
The movie uses the character to poke a little fun at our society. If we hear these ideas coming from a homeless man, we think he's crazy. If we read the ideas in a 1,000-year-old book and millions of people believe it, then we enshrine them as religion.
There's also a little mystery surrounding the street evangelist. The movie hints that he may be a time-traveler but never directly says so.
When he spies Cole in the streets, he shouts out to him, "You. You! You're one of us!" What does he mean by this? Does he mean that he recognizes Cole as a citizen of the future? Or perhaps only that he recognizes Cole as a fellow mentally divergent? We never find out.
As an added wrinkle, the street evangelist looks a lot like the 14th-century doomsayer from Railly's presentation. Said doomsayer used "unfamiliar words and [spoke] in a strange accent" and "made dire prognostications about pestilence, which he said would wipe out humanity in approximately 600 years." These characteristics would fit a time-traveler and line up with the street evangelist's sermon, too.
Or we could chalk it up as huge coincidence. What do you think?