What person does the point of departure between Christianity and the other two religions revolve around?

It not a person, it's just that Christianity made up so many beliefs. Baptism was borrowed from the Jewish religion, it started as the Jewish Custom of bathing before prayer service. Mary never ascended into heaven, it was only a legend until a 7th Century Pope decreed that it really happened and if you didn't believe that, reprisals were next. And the list goes on.