The big problem is, when the leader of these cults dies or gets whacked, the following dies then or soon thereafter. The gospel truly has a life of it's own, or should I say the life of God in it. Otherwise, it would have died out like the cults you mention.
The reason Christianity and Islam continued so long is each was useful to a evolving State.
Sadly this is partially true.
I tend to believe that Christianity lost an awful lot when it gained worldly prominence, as is well illustrated by the historically correct accusations hurled in this thread against the church. Power does indeed corrupt, and absolute power corrupts absolutely. No where have we seen this truth illustrated more powerfully than in the church for many reasons but chiefly that our Savior repudiated and refused such power even as He hung on a cross.
Of course your theory on Christianity's survival falls apart in areas of the world where the state has suppressed its message and oppressed its adherents. The Church thrives where the blood of saints is spilled.
