Not everyone recognizes Jesus’ authority; others sense the power but do not respond with faith. Even some who naturally belong to the kingdom, that is, the Jews who had lived under the old covenant and had been the heirs of the promises, turn out to be rejected. They too approach the great hall of the messianic banquet, lit up with a thousand lamps in joyous festivity; but they are refused admission, they are thrown outside into the blackness of night, “where there will be weeping and gnashing of teeth” (8:12). The idea is not that there will be no Jews at the messianic banquet. After all, the patriarchs themselves are Jews, and all of Jesus’ earliest followers were Jews. But Jesus insists that there is no automatic advantage to being a Jew. As he later says to those of his own race, “Therefore I tell you that the kingdom of God will be taken away from you and given to a people who will produce its fruit” (21:43). An individual’s faith, his or her response to the authority claims of Jesus, will prove decisive. The alternative to entrance into the kingdom is painted in horrible colors: literally the weeping and the gnashing of teeth, to emphasize the horror of the scene, the former suggesting suffering and the latter despair. The same authority of Jesus that proves such a great comfort to the eyes of faith now engenders terror in the merely religious.
This is not a teaching that is very acceptable to vast numbers in western Christendom today. It flies in the face of the great god Pluralism who holds much more of our allegiance than we are prone to admit. The test for religious validity in this environment is no longer truth but sincerity—as if sincerity were a virtue even when the beliefs underlying it are entirely mistaken.
– D.A. Carson, Jesus’ Sermon on the Mount and His Confrontation with the World, 166
(HT: Zach Nielsen)