Dean finds out that Zachariah found him because he had some of the "fringe Christian" groups on the lookout for him. When Castiel asks Dean how he was found Dean replies "long story, let's just stay away from Jehovah's Witnesses". The minister that spotted Dean in front of his motel was handing out brochures about the Rapture, which is a belief among some Christian denominations that all faithful Christians will rise into the sky and join Christ in heaven at the end of days, and that non-believers will be "left behind" on Earth and will have to try and live through Armageddon, the war between Heaven and Hell, which is fought on Earth. However Jehovah's Witnesses do not believe in the Rapture, they believe that only a very select few chosen ones (144,000 specifically) will go to heaven and that the vast majority of faithful Christians will live forever in perfection on a paradise Earth.
In the future segment of the episode, the world has come to an end and the infrastructure in the cities has been destroyed. But during Cass and Dean's conversation in the car, traffic lights can be clearly seen in the background.