In Revelation Chapter 7, why was Dan the only one of the ten tribes of Israel that was excluded or set aside, since all ten tribes fell into idol worship?
In order to address why the tribe of Dan was symbolically set aside in the image of the 144,000 people marked in heaven as the "new Israel", we need a little history about the 12 tribes of Israel. Bear in mind that the audience during the time of Revelation knew the history of Israel and would instantly see the meaning behind the omission of the tribe of Dan.
In 930 BC the royal kingdom of God's chosen people, which consisted of the twelve tribes of Israel, was divided into two separate kingdoms: Israel and Judah. Israel, consisting of 10 tribes, was in the north and ruled by King Jeroboam. And, Judah which was made up of the tribes of Judah and Benjamin was in the south and ruled by King Rehoboam. The center of worship for both kingdoms, the temple, was in Judah in the south. This posed a problem for Jeroboam, the king of Israel; for he knew that he could lose the loyalty of his people if they continued to worship in the temple in Jerusalem. Fearing a reunification of the twelve tribes of Israel, Jeroboam's solution was to make a parallel mock religion. He instituted his own priesthood, feast days, and high places, and made two golden clave (false idols) for the people to worship, and said to the people, "You have been going up to Jerusalem long enough. Here is your God, O Israel, who brought you from the land of Egypt.' And he put one (golden calf) in Bethel and one in Dan. This led to sin, because the people frequented these calves in Bethel and in Dan" (1 Kings 12:28-30).
With the placement of the golden calf in the northern most boundary of Israel, the tribe of Dan completely lost its identity and fell into idolatry. So extensive was their worship of a false god that the tribe itself eventually disappeared. This idolatry, the bestowing on a creature the reverence due to God alone, is in itself the greatest of mortal sins. And with the gravest sin, comes the gravest punishment 'eternal separation from God. Those reading the book of Revelation would have received the message loud and clear and known why the tribe of Dan was not counted among the 144,000 protected by the seal of God; for those who choose to worship a false god, choose their own demise.