Matthew 10:6

6.But go rather to the lost sheep The first rank, as we have said, is assigned to the Jews, because they were the firstborn; or rather, because at that time they alone were acknowledged by God to belong to his family, while others were excluded. (569) He calls them lost sheep, partly that the apostles, moved by compassion, may more readily and with warmer affection run to their assistance, and partly to inform them that there is at present abundant occasion for their labors. At the same time, under the figure of this nation, Christ taught what is the condition of the whole human race. The Jews, who were near to God, and in covenant with him, and therefore were the lawful heirs of eternal life, are nevertheless pronounced to be lost, till they regain salvation through Christ. What then remains for us who are inferior to them in honor? (570) Again, the word sheep is applied even to the reprobate, who, properly speaking, did not belong to the flock of God, because the adoption extended to the whole nation; as those who deserved to be rejected, on account of their treachery, are elsewhere called the children of the kingdom, (Matthew 8:12.) In a word, by the term sheep, Christ recommends the Jews to the apostles, that they may dedicate their labors to them, because they could recognize as the flock of God none but those who had been gathered into the fold.