[Ali Imran]

Christ's Death and Resurrection is central to all of Christianity.  He is also The Son of God......this was revealed at Jesus' baptism.  "Behold this is my beloved Son!"

Ali will repeat ad infinitum "Son of Mary" and not say "Son of God"  Indeed, "son of Mary" can be found in the Bible but it was said in a different context, which Ali gingerly ignored to mislead those who didn't know the context.

The son of God title is not unique to Jesus. Many were given the same title, like King David and others. 

But son of Mary is unique to Jesus. Nobody else has such a name, where the mother's name is used to identify the person. Unique only to him.