Four more speculations. 
I argue that an important element of Christianity’s success was that it quickly transformed itself into what I call the imperial church incorporate. Would Christianity have succeeded had it not done so? 
Second, could Mithraism have triumphed over the church? 
Third, could the Gnostic variants of Christianity come to dominate? 
Finally, the Jewish arm of the church vanished after the Bar Kosiva revolt of 132. What might have happened had that group survived and thrived?