Jamieson, Fausset and Brown on Matthew 10:6

6. But go rather to the lost sheep of the house of Israel--Until Christ's death, which broke down the middle wall of partition (Ep 2:14), the Gospel commission was to the Jews only, who, though the visible people of God, were "lost sheep," not merely in the sense which all sinners are (Is 53:6; 1Pe 2:25; compare with Lu 19:10), but as abandoned and left to wander from the right way by faithless shepherds (Jr 50:6, 17; Eze 34:2-6, &c.).