The Four Horsemen
The book of Revelations speaks of end times and the inevitable "end of the world". This end has different interpretations of what it really means. A couple of them are; complete destruction of the earth and its inhabitants, the end of sin and removal of all sinners on earth or just the end of the world as we currently know it. The initial step leading to the Apocalypse is the breaking of the Seven Seals. The majority of people who interpret the opening of the seals believe that they are opened by the Lamb, who is "our Lord Jesus in resurrection glory" (http://www.revelation-today.com/Seal1st2.htm).
The belief that these seals were opened under the power of God is shown with the Lamb returning to take what is rightfully his. "The Lamb, whose right it is, has received the earth's title deed and now must begin to take possession of His kingdom" (Morris, pg. 107), shows us that Jesus has returned and will cleanse his kingdom of sinners to become a place of peace and prosperity. There are seven seals that lead to the Apocalypse, however, the first four will be the focus of this section. The first four seals are the four horsemen of the Apocalypse. These four horses "cover each direction of the compass" (http://www.apocalipsis.org/fourhorsemen.htm).
The four horsemen are represented by conquest, war, famine and death.
In the book of Revelations, it is said that seven seals are to be broken to bring on the Apocalypse. In Supernatural, Lilith is a demon attempting to bring forth Lucifer by breaking seals. There are hundreds of seals that she can break, with the initial being mentioned earlier as Dean accepted the offer to torture souls in hell. She must break 66 seals, including the first, to have Lucifer walk. This is different than the seven seals mentioned in Revelations but what happens when the seals are opened also differs. However, the fact that seals are to be broken in Supernatural, shows religious insights to the show.
Supernatural also deals with the four horsemen of the Apocalypse. As of now, at least two of the four have been featured in the series (War and Famine) and a third could be argued was shown (Death). They are depicted in a more modern sense than back in the book of Revelation.