And I say unto thee, Let my son go. This was not the beginning of the legation, but its final clause; for Moses warned the desperate man of his son's death, when everything else had been tried in vain. The meaning is, then, that the obstinacy of the tyrant must not prevent Moses from pressing him even to this final act. Therefore this injunction was an exhortation to perseverance; as appears from the context, when God declares that he will punish the obstinacy of the tyrant, because he refused to obey the command to let the people go. Moreover, since this denunciation was very severe, and might very greatly awaken the tyrant's wrath, therefore Moses is thus early commanded to prepare himself lest he should fail in this particular.

Let my son go, that he may serve me - Which they could not do in Goshen, consistently with the policy and religious worship of the Egyptians; because the most essential part of an Israelite's worship consisted in sacrifice, and the animals which they offered to God were sacred among the Egyptians. Moses gives Pharaoh this reason Exodus 8:26.
I will slay thy son, even thy first-born - Which, on Pharaoh's utter refusal to let the people go, was accordingly done; see Exodus 12:29.

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.

Let my son go - Not only my servant whom thou hast no right to detain, but my son whose liberty and honour I am jealous for. If thou refuse, I will slay thy son, even thy first - born - As men deal with God's people, let them expect to be themselves dealt with.