4 But of the one sheepfold and of the one Shepherd, you are now indeed being constantly reminded; for we have commended much the one sheepfold, preaching unity, that all the sheep should enter by Christ, and none of them should follow Donatus. Nevertheless, for what particular reason this was said by the Lord, is sufficiently apparent. For He was speaking among the Jews, and had been specially sent to the Jews, not for the sake of that class who were bound up in their inhuman hatred and persistently abiding in darkness, but for the sake of some in the nation whom He calls His sheep: of whom He says, “I am not sent but to the lost sheep of the house of Israel.” He knew them even amid the crowd of His raging foes, and foresaw them in the peace of believing.
What, then, does He mean by saying, “I am not sent but to the lost sheep of the house of Israel,” but that He exhibited His bodily presence only to the people of Israel? He did not proceed Himself to the Gentiles, but sent: to the people of Israel He both sent and came in person, that those who proved despisers should receive the greater judgment, because favored also with the sight of His actual presence. The Lord Himself was there: there He chose a mother: there He wished to be conceived, to be born, to shed His blood: there are His footprints, now objects of adoration where last He stood, and whence He ascended to heaven: but to the Gentiles He only sent.
Source: Tractates on the Gospel of John (New Advent)