This promise ought not to make us think of a mass conversion of Jews to Christianity or of all Jews on the last day acknowledging that Christians were right after all (though that will certainly happen). This promise points us to the fact that some from that synagogue of Satan
will repent and embrace the gospel. An example (though not from Philadelphia) would be the apostle Paul.
9 Behold, I will make those of the synagogue of Satan who say that they are Jews and are not, but lie behold, I will make them come and bow down before your feet, and they will learn that I have loved you.