Because of the Messianic prophecies in the OT, ANYONE who claims to be the Messiah will have to PROVE that he is from the tribe of Judah and the line of David.
Phew, sorry for the long delay. A lot of searching went into this one!
I couldn't find any biblical reference stating that the Messiah *HAD TO PROVE* his genealogy, only that he had to be descended from that line. What I did find was rather interesting, as follows:
In Jewish law, lineage is determined through the father, not the mother and belief was the Messiah would 'be of this Earth'. It is therefore impossible for Jesus to have been descended from the line of David as he was supposed born of a virgin birth. Without an Earthly father and a clear line of descendant, how can he 'prove' to be of that line by Jewish law? We still are left with the added problem that the Second Temple has not been built to the exactness as it should have been built and thus has not been destroyed. We are also given two conflicting views of Jesus' supposed lineage, in Mathew's account, Jesus could not be the Messiah as he was descended from Jechonias, in which God had cursed this line of lineage.
Seeing as how Jesus was of the house of Judah, but of cursed lineage and still attempting to preach the word of peace, perhaps when God was speaking to him he was telling Jesus about the Christians that would come. Jesus is claimed to be Jewish, but by Jewish law, he is not Jewish and of cursed lineage meaning that anyone whom worships at his feet would be Jews that are not Jews. God talks to a lot of people who are not the Messiah, so it stands to reason that there should be no doubt that this is still the case with Jesus when God is telling him all about Christianity and how they will worship at his feet as a false prophet.
I just realized we are also talking on another Thread 
