Exodus 4:23

and I say unto thee, Let My son go that he may serve me; and if thou refuse to let him go, behold, I will slay thy son, even thy first-born. This threat looks forward to the last of the Egyptian plagues. The fact that Israel is called God's first-born son suggests, even here, that the Lord would later choose others, that He would gain spiritual children out of the heathen nations.