A couple of notes on Mertseger's stellar writeup:
  • This reference to the idea of a good God opening the door to the existence of a Satan is perhaps best approached using the old sense of the word "Satan" - Hebrew for adversary. A monotheistic religion may well believe in an opposing force of evil, but will by necessity view it as inferior to God. By contrast, dualistic religions (such as Zoroastrianism) tend to believe that the two gods of good and evil are of equal power, though neither is strictly speaking omnipotent.
  • While monotheists may believe in the existence of lesser supernatural beings such as angels, and may even accept the existence of entities that are worshiped as gods by others, they will not consider them to be gods. (I'm specifically thinking of C.S. Lewis's Space Trilogy, in which he portrays the members of the Greco-Roman pantheon as angels mistaken for gods by ancient peoples.) The difference is most discernable in the question of worship - Christians and Muslims may believe in angels, but they would consider it a grievous sin to worship one since in their theology only God is worthy of worship.

    I'm not qualified to comment on the issue of whether or not Roman Catholics worship saints, so I won't. Maybe some Catholic Everythingian could chime in and explain how they figure in the practice of their faith?