They are to enact signs from the Book of Revelation to warn people of the coming End of Days, while bringing about the End of the Old World.
Professor Gellar did not believe that he was one of the two witnesses and considered Travis delusional. Shortly thereafter, Gellar was stabbed by Travis with the Sword of John the Revelator, which killed him instantly. Travis blocked out this memory after hiding Gellar's body in a church's freezer. He envisioned Professor James Gellar once more as an illusion, this time cooperative with their cause.
As one of the two witnesses, Travis enacted what were referred to as the Doomsday Tableaus where a human sacrifice is required for each picturesque crime scene. According to notes taken by the original Professor Gellar, the creation of these events will trigger the End of Days and bring about the New World.