Bridegroom of Blood

Rabbinic commentary on this passage holds that Jethro (Moses' father-in-law, and the "Priest of Midian") was not a worshiper of YHWH. The Rabbis are not fully in agreement about when or whether Jethro converted to Judaism, but he did bless God after the Exodus (Exodus 18:10).

This may explain why Moses' son hadn't been circumcised, and why Moses' wife was reluctant to do it. She eventually did, apparently to keep God from killing Moses in His anger.

10 But Moses said to the Lord, “Oh, my Lord, I am not eloquent, either in the past or since you have spoken to your servant, but I am slow of speech and of tongue.” 11 Then the Lord said to him, “Who has made man's mouth? Who makes him mute, or deaf, or seeing, or blind? Is it not I, the Lord? 12 Now therefore go, and I will be with your mouth and teach you what you shall speak.” 13 But he said, “Oh, my Lord, please send someone else.” 14 Then the anger of the Lord was kindled against Moses and he said, “Is there not Aaron, your brother, the Levite? I know that he can speak well. Behold, he is coming out to meet you, and when he sees you, he will be glad in his heart. 15 You shall speak to him and put the words in his mouth, and I will be with your mouth and with his mouth and will teach you both what to do. 16 He shall speak for you to the people, and he shall be your mouth, and you shall be as God to him. 17 And take in your hand this staff, with which you shall do the signs.”

Moses Returns to Egypt

18 Moses went back to Jethro his father-in-law and said to him, “Please let me go back to my brothers in Egypt to see whether they are still alive.” And Jethro said to Moses, “Go in peace.” 19 And the Lord said to Moses in Midian, “Go back to Egypt, for all the men who were seeking your life are dead.” 20 So Moses took his wife and his sons and had them ride on a donkey, and went back to the land of Egypt. And Moses took the staff of God in his hand (Exodus 4:10-20).

21 And the Lord said to Moses, “When you go back to Egypt, see that you do before Pharaoh all the miracles that I have put in your power. But I will harden his heart, so that he will not let the people go. 22 Then you shall say to Pharaoh, ‘Thus says the Lord, Israel is my firstborn son, 23 and I say to you, “Let my son go that he may serve me.” If you refuse to let him go, behold, I will kill your firstborn son.’”

Bridegroom of Blood

24 At a lodging place on the way the Lord met him and sought to put him to death. 25 Then Zipporah took a flint and cut off her son's foreskin and touched Moses' feet with it and said, “Surely you are a bridegroom of blood to me!” 26 So he let him alone. It was then that she said, “A bridegroom of blood,” because of the circumcision (Exodus 4:21-26).

There remains one glaring difficulty with this passage: God sent Moses to Egypt on a mission. God told Moses that the Egyptians who wanted to kill him were dead now. Then God sought to kill him before he got to Egypt.

Explanation number one:
Moses could go to Egypt, talk to Aaron, help Aaron liberate Israel "as God to him" -- all after having been killed by God.

Explanation number two:
Because Moses' son wasn't circumcised (wasn't a Jew, wasn't in covenant with God) he would have been killed by God with the Egyptian sons. Moses couldn't win the argument with Zipporah to circumcise his son, so he decided not to go to Egypt after all. Zipporah only relented when it was clear that Moses was going to die for his disobedience.