4:19. And the Lord said to Moses, in Madian: Go, and return into Egypt; for they are all dead that sought thy life.
4:20. Moses therefore took his wife, and his sons, and set them upon an ass; and returned into Egypt, carrying the rod of God in his hand.
4:21. And the Lord said to him as he was returning into Egypt: See that thou do all the wonders before Pharao, which I have put in thy hand: I shall harden his heart, and he will not let the people go.
I shall harden, etc... Not by being the efficient cause of his sin; but by withdrawing from him, for his just punishment, the dew of grace that might have softened his heart; and so suffering him to grow harder and harder.
4:22. And thou shalt say to him: Thus saith the Lord: Israel is my son, my firstborn.
4:23. I have said to thee: Let my son go, that he may serve me, and thou wouldst not let him go: behold I will kill thy son, thy firstborn.
4:24. And when he was in his journey, in the inn, the Lord met him, and would have killed him.
The Lord met him, and would have killed him... This was an angel representing the Lord, who treated Moses in this manner, for having neglected the circumcision of his younger son; which his wife understanding, circumcised her child upon the spot, upon which the angel let Moses go.
4:25. Immediately Sephora took a very sharp stone, and circumcised the foreskin of her son, and touched his feet, and said: A bloody spouse art thou to me.
4:26. And he let him go after she had said: A bloody spouse art thou to me, because of the circumcision.