Thou shalt say unto Pharaoh, Israel is my son. This would be addressing Pharaoh in language familiar to him. Each Egyptian monarch of this period was accustomed to style himself, "son of the Sun," and to claim and expect the constant favour and protection of his divine parent.
It was also quite within the range of Egyptian ideas that God should declare himself by word of mouth to his special favourites, and give directions as to their actions. My firstborn. Not only "as dear to me as to a father his firstborn" (Kalisch), but the only nation that I have adopted, and taken into covenant, so as to be unto me "a peculiar people above all the nations that are upon the earth" (Deuteronomy 14:2).
Israel's sonship is here mentioned for the first time.