Jesus calls the Jews lost sheep
because their nation is under divine judgment for rejecting Jesus as their Shepherd. Since the people of Abraham, whom John the Baptist prepared for the coming of the Lord, do not allow themselves to be led by the Son of God, then of all people in the world, this people are in the greatest distress and danger: lost sheep, wandering without a shepherd (see Matthew 9:36).
From their perspective, the Jews would be much more comfortable with characterizing the Samaritans and Gentiles as lost sheep, with Israel as the flock being fed by the shepherds of the world. As such, it would come to them as a great shock that Jesus would refer to the Lord’s own flock as the lost sheep. This characterization points to a divine judgment that applies to the entire nation, not just the lost sinners in Israel. This much is clear from the contrast between Gentiles
on the one hand, and "house of Israel” on the other. Why did the house of Israel become a lost flock? This is the result of the people failing to recognize Jesus’ voice and not being able to follow this Shepherd with a true faith. If the people of Abraham, whom John the Baptist prepared for the coming of the Lord himself, do not allow themselves to be led by the Son of God, then it would mean that of all the people of the world, this people would be in the greatest distress and danger. They are lost sheep, who wander around without a shepherd (see Matthew 9:36)1
5 These twelve Jesus sent out, instructing them, “Go nowhere among the Gentiles and enter no town of the Samaritans,