WHY  CHRISTIANITY  SUCCEEDED???

        It is one of those questions whose answer that is rather straightforward for a person who has read widely the works of the classical world.  It wasn't the message, Gods will, the absurdity of the pagan beliefs,* but rather a combination of very apparent factors.

1.  Mystery religions were popular with the masses; however, they tended to be quite exclusive.  They required the learning of esoteric dogma, which required of the believer a verbal and intellectual skill which was missed by the illiterate masses.  Thus these religions (the worship of Isis, Gnosticism, and Mithraism) didnt attempt to build a following among the illiterate masses, and over half the people were illiterate. 

2.  Judaism had a racial qualification.

3.  Paganism was the only competing mass religion.  Though it had state support, it didnt have an effective organization and leadership.  Moreover, they were splintered, each temple was an island and each cult of a god was also an island.  The priesthood (with very few exceptions) were on a whole closer to temple servants who sometimes led services.  The pagan mystery cults held their ceremonies mainly at sacred locations and thus were often in remote areas.  

4.  Paganism didnt promise the reward of heaven (Elysium Fields) for the faithful mases.   

5.  The state, which built temples, was tolerant of other religions.**  

6.  The philosophies, Neoplatonic, Stoic, and Epicurean, had significant followings among the educated.  By attracting the best mind weakened the support of the Olympian Gods. 

7.  The Hebrews had a main temple (and several others that formed principle centers of worship) and a high priest a large body of supporting rabbis.  The Christians emulated the Jews in build an organization with a bishop in each center of worship and a large supporting priesthood.  The structure with its Pope permitted the formation of an organization that could succor government support, attack heretics, and do various other acts necessary to promote its cause. 

*  The Christian beliefs were on an equal footing.  This is admitted by Justin Matyr:

And when we say also that the Word, who is First begotten of God, was born for us without sexual union, Jesus Christ our teacher and that He was crucified and died and rose again and ascended into heaven, we propound no9thing new beyond what you believe concerning those whom you call sons of Zeus.  For you know of how many sons of Zeus your esteemed writers speak:  Hermes the interpreting Word and teacher of all; Asclepius, who thought he was a great healer, after being struck by a thunderbolt ascended into heaven; and Dionysus too who was torn to pieces; and Heracles, when he had committed himself to the flames to escape his pains; and the Dioscuri, the sons of Leda; and Perseus, son of Danae; and Bellerophon, who though of mortal origin rose to heave3n on the horse Pergasus.  For what shall I say of Ariadne, and those who, like her, have been said to have been placed among the stars?  And what of your deceased emperors, whom you think it right to deify and on whose behalf you produce someone who swears that he has seen the burning Caesar Ascend to heaven from the funeral pyre?  The First and Second Apologies, in Ancient Christian Writers vol. 56, New York; Paulist Press, Ch. 21.

**  The claims by Christians of persecution are grossly extraverted by the Christiansm.  What little documented repression existed, it was a response to the Christian opposition to Paganism.   For example, they didn't deny the existance of the pagan Gods, but rather said that they were devils.  They refused to partake is national holidays, and generally they did not join the military.  When Christianity became the state religion, they passed a series of laws to wipe out paganism--and other religions.  A Catholic theologian write of these parallels at  http://skeptically.org/newtestament/id11.html .