It is impossible for it to spread at the same rate.
Judaism believes that experiences of God are tribal -- it takes for granted the notion that non-Jews can be righteous on their own cultural terms and therefore tries to disuade perspective converts instead of enticing them to join.
In the early middle ages there were a number of Jewish-ruled kingdoms (in Africa, Persia, eastern Euroe) -- and it all cases, contrary to Christian practice, they had a legal system which didn't just tolerate different religions but fully accomodated them.
That said, if the Empirial family had not accepted Christianity at the time of Constantine, or if the anti-Christian emperor Julian had not been assassinated, Christianity may not have become the dominant force that it did.
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Interestingly, if Julian had not been assassinated, the temple would have been rebuilt and Israel would have been re-instituted as a Jewish state.
This would have caused a ... re-constriction of the Jewish worldwide presence
Definitely a very different world -- but certainly not one where Judaism would have dominated.