Yahweh told Moses, “When you arrive back in Egypt, go to Pharaoh and perform all the miracles I have empowered you to do. But I will harden his heart so he will refuse to let the people go. Then you will tell him, ‘This is what Yahweh says: Israel is My firstborn son. I commanded you, “Let my son go, so he can worship Me.” But since you have refused, I will now kill your firstborn son!’”
On the way to Egypt, at a place where Moses and his family had stopped for the night, Yahweh confronted Moses and was about to kill him. But Moses’ wife, Zipporah, took a flint knife and circumcised her son. She touched his feet with the foreskin and said, “Now you are a bridegroom of blood to me.” (When she said “a bridegroom of blood,” she was referring to the circumcision.) After that, Yahweh left Moses alone. (Ex. 4:21-26)
Moses was a Jew. Jews knew that they were descendants of Abraham, Isaac and Jacob. They also knew that the God of their ancestors (Yahweh) gave special promises to Abraham’s descendants–promises which included gaining residence in some lush patch of land (which was called the Promised Land or Canaan). Yahweh said these promises were part of a Covenant, and that all of His male followers had to get circumcised as a means of declaring their association with Him under that Covenant.
Circumcision (which means “to cut around”) is a removal of the foreskin on a man’s penis (foreskin is a flap of skin which covers the tip of the penis from view). It does not damage the penis’ ability to function in any way, but it does change the appearance of the end of the penis—a fact which some use to argue that the whole ritual is a gross disfigurement.
It’s important to note that Yahweh was not requiring a procedure that would permanently cripple men. While getting circumcised is certainly a painful process, it also heals fairly quickly (7-10 days), and it has some health benefits when it comes to keeping that part of the body clean. Whenever possible, Yahweh ordered infant boys to be circumcised when they were eight days old. It’s interesting to note that after 14 days, infants find the procedure significantly more painful and traumatizing, so God intentionally ordered the circumcising to happen within a period of time when He knew there would be much less pain and no memory of the event.
Since circumcision served as a sign of one’s association with Yahweh, it was a very big deal—so much so that Yahweh commanded anyone who refused to participate to be cut off from Israel.
In Exodus 4, Moses has just had a personal encounter with Yahweh (the famous burning bush). Moses (who was a Levite) would have been circumcised as an infant by his Jewish parents, however he has not circumcised his son. This puts him in violation of Yahweh’s Covenant rules (the Covenant that Yahweh had made with Abraham). Moses is the author of Exodus, and he doesn’t share his reasons for not circumcising his son. Was he just being lazy? Did his non-Jewish wife object to the idea? Whatever the reason, the wife clearly understands that Yahweh demands circumcision, because she rushes to do it when God’s angry Presence appears in some form to her and Moses during their travels to Egypt. Zipporah doesn’t sound like a fan of the procedure with the comment she gives afterwards, however she does show respect for Yahweh by aligning with His requirements.
So why was God so angry with Moses? Because as the head of the household, He holds Moses responsible for making sure this duty is done. God is rightfully ticked that after Moses has had such a personal encounter with God, he’s still blowing off one of God’s well-known requirements for the Jews. Moses’ attitude needs some adjusting here. He was giving Yahweh flack at the burning bush scene earlier in this same chapter, and now he’s shirking in his duties as the spiritual leader of the home. Happily, Zipporah steps in and saves him from God’s wrath by doing what Moses had left undone.