Hmm, maybe:
Eliminate Paulus.
With Christians / Nazarenes being one Jewish group among others (Saducees, Pharisees, Essenes, John-the-Baptist`s-followers etc. being others), they would still be "united" under one umbrella.
I don`t know how to avoid butterflying Islam given this, though. You could have some Arabian religious reformer create yet another offshoot that`d also consider itself a branch of Judaism. If you`re satisfied with that, that may be achievable.
Other than that... With Christianity being Roman state religion etc. and Islam having emerged with its Caliphate, it´s all so distinct and political that, as
@Achaemenid Rome has pointed out, any syncretist attempt would only result in the creation of a new sect, but wouldn`t be able to absorb everyone. Not even by far.