Matthew 15:24

I am not sent but unto the lost sheep - By the Divine appointment, I am come to preach the Gospel to the Jews only. There are certain preachers who should learn a lesson of important instruction from this part of our Lord's conduct. As soon as they hear of a lost sheep being found by other ministers, they give all diligence to get that one into their fold: but display little earnestness in seeking in the wilderness for those that are lost. This conduct, perhaps, proceeds from a consciousness of their inability to perform the work of an evangelist; and leads them to sit down in the labors of others, rather than submit to the reproach of presiding over empty chapels. Such persons should either dig or beg immediately, as they are a reproach to the pastoral office; for, not being sent of God, they cannot profit the people.

The wilderness of this world is sufficiently wide and uncultivated. Sinners abound every where; and there is ample room for all truly religious people, who have zeal for God, and love for their perishing follow creatures, to put forth all their strength, employ all their time, and exercise all their talents, in proclaiming the Gospel of God; not only to the lost sheep of the house of Israel, but to a lost World. Nor can such exertions be unsuccessful. There the pure truth of God is preached, many will be converted. Where that truth is preached, though with a mixture of error, some will be converted, for God will bless his own truth. But where nothing but false doctrine is preached, no soul is converted: for God will never sanction error by a miracle of his mercy.

But he answered and said, I am not sent ... - This answer was made to the woman, not to the disciples.

The "lost sheep of the house of Israel" were the Jews. He came first to them. He came as their expected Messiah. He came to preach the gospel himself to the Jews only. Afterward it was preached to the Gentiles, but the ministry of Jesus was confined almost entirely to the Jews.

15:24 I am not sent - Not primarily; not yet.