(23) I will slay thy son, even thy first-born.--The threat was not made until immediately before the tenth plague (Exodus 11:5). It is not recorded in the words which Moses is here directed to use; but the speech of Moses in Exodus 11 is no doubt much abbreviated.

Verse 23. - I will slay thy son, even thy firstborn. For the fulfilment of the threat, see Exodus 12:29. Moses did not utter it till all other arguments were exhausted, and he knew that he was having his last interview with the monarch (Exodus 10:29; Exodus 11:4, 5). In this reserve and in the whole series of his dealings with the Egyptian king, we must regard him as simply carrying out the special directions which, after his return to Egypt, he continually received from the Almighty. (See Exodus 6:11; Exodus 7:9, 15 19: 8:1, 5, 16, 20, etc.)

CHAPTER 4:24-26

4:18-23 After God had appeared in the bush, he often spake to Moses. Pharaoh had hardened his own heart against the groans and cries of the oppressed Israelites; and now God, in the way of righteous judgment, hardens his heart against the teaching of the miracles, and the terror of the plagues. But whether Pharaoh will hear, or whether he will forbear, Moses must tell him, Thus saith the Lord. He must demand a discharge for Israel, Let my son go; not only my servant, whom thou hast no right to detain, but my son. It is my son that serves me, and therefore must be spared, must be pleaded for. In case of refusal I will slay thy son, even thy first-born. As men deal with God's people, let them expect so to be dealt with.And I say unto thee, let my son go, that he may serve me,.... Worship God according to his will in the place he had designed for him, and where he might be safe and free; and which service was due from him as a son, and to be performed not in a servile way, but in a filial manner, and therefore as a servant he could demand his dismission, and much more as his son; and this is required in an authoritative way, for saying is here commanding, insisting on it as a point of right to be done:

and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn; meaning, not only in a strict and literal sense Pharaoh's firstborn son, and heir to his crown, but the firstborn of all his subjects, which in a civil sense were his. This was not to be said to Pharaoh at the first opening of his commission to him, but after all methods had been tried, and the several other plagues designed were inflicted on him to no purpose, he was to be told this, which was the last plague, and succeeded; but this is told to Moses before hand, that when other messages he should be sent with to him, and all that should be done by him would prove ineffectual, this, when sent with and performed, would have the desired effect.