Many fantastic interpretations have been given, but none have been proved. The vast majority of interpreters regard the number as a mystical symbol, designating some man who will be the chief agent of Satan towards the end of the world. Some people thought it was Mahomet, saying that he died in 666 A.D. But he died in 630 A.D. Calvin wished to attribute it to Pope Boniface III., or to the Popes in general. His only foundation was prejudice, and his theory is utterly rejected to-day. Martin Luther’s name, and dozens of others have been made to signify the number in various languages, but in all these cases the wish was father to the thought, and was made to supply for the lack of reason. The true solution of this question cannot be given.
The Pope, Question 346 from Radio Replies Volume I Copyright © 1938 (by Rev. Dr. Leslie Rumble, M.S.C. and Rev. Charles Mortimer Carty).