Exodus 4:22 Then you shall say to Pharaoh, ‘Thus says the LORD: "Israel is My son, My firstborn. 23 "So I say to you, let My son go that he may serve Me. But if you refuse to let him go, indeed I will kill your son, your firstborn." 24 And it came to pass on the way, at the encampment, that the LORD met him and sought to kill him. 25 Then Zipporah took a sharp stone and cut off the foreskin of her son and cast it at Moses’ feet, and said, "Surely you are a husband of blood to me!" 26 So He let him go. Then she said, "You are a husband of blood!" —because of the circumcision.
God sought to kill Moses because he had failed to circumcise one of his sons. God had made a covenant with Abraham of which circumcision was the sign. Every male child was to be circumcised at eight days old.
Genesis 17:11 ...and you shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between Me and you. 12 "He who is eight days old among you shall be circumcised, every male child in your generations, he who is born in your house or bought with money from any foreigner who is not your descendant. 13 "He who is born in your house and he who is bought with your money must be circumcised, and My covenant shall be in your flesh for an everlasting covenant. 14 "And the uncircumcised male child, who is not circumcised in the flesh of his foreskin, that person shall be cut off from his people; he has broken My covenant.
In failing to circumcise his son, Moses had condemned him to be cut off from God’s covenant with Abraham.
God’s covenant with Abraham promised a future resurrection to an eternal inheritance of the land and thus eternal life.
Now Moses was being sent to Pharaoh to give him an ultimatum to let Israel, whom God calls His ‘firstborn’, go free to serve God.
Moses was thus guilty of hypocrisy in that by neglecting to circumcise his son he was preventing him from serving God, placing him outside the covenant.
Before he could tell Pharaoh to let God’s firstborn (Israel) go free to serve God, Moses must first free his own son to serve God by circumcising him.
We are not told why Moses had failed to circumcise his son, maybe Zipporah had been against the idea. She certainly understood what the problem was and how to resolve it.
I hope you find this helpful.