To show continuity from the geographical Israel to the Catholic Church, the true Israel.
Either Jesus had a hidden agenda and could not give it out (it would be blasphemy) or this is something the Church made up, as part of the deliberate 'transformation' of Jewish Christianity into Pauline Christianity, Alex.
Reading the Gospels, Jesus is unequivocal about the purpose of why and for whom he was sent, and what ethnic limitations his mission entailed.