Exodus 4:21-26 “And the LORD said to Moses, "When you go back to Egypt, see that you do all those wonders before Pharaoh which I have put in your hand. But I will harden his heart, so that he will not let the people go. "Then you shall say to Pharaoh, 'Thus says the LORD: "Israel is My son, My firstborn."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." And it came to pass on the way, at the encampment, that the LORD met him and sought to kill him. 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!" So He let him go. Then she said, "You are a husband of blood!"-- because of the circumcision.

We see Israel as a nation is God’s firstborn, one who has preeminence. The Father, is Begetter of Israel (Deut. 32:18; Jer. 2:27; Isa. 44:8; Mal. 1:6, 2:10), there is a paternal relationship as the Lord trained His son into be a holy nation by love and discipline of a Father.

God has Moses to specifically say he will kill Pharaohs first born son, if he does not free his Son, the people of Israel. How can Moses say such a statement of judgment to Pharaoh when he was disobedient. This was part of the Abrahamic covenant for all Jewish males. Genesis 17:9-14 this covenant with Abraham was an eternal one, it was to be kept from generation after generation. And God states that any person who does not do its requirement of circumcision, “that person will be cut off from his people, because he has broken my covenant.”

Moses knew better and neglected the circumcision of his son which was to be done on the eighth day after birth, as God commanded in the Scripture. Moses was not just being disobedient as the head of his household, but as the head of the nation of Israel. Moses had probably neglected circumcision in regard to his Midian wife, who detested the operation.

God comes to Moses, seeking to take his life, as he rests before his journey to Pharaoh. Zipporah who was not an Israelite knew what Moses disobedience was. She knew circumcision was a sign of the covenant and so she took the initiative and did it herself and chastised her husband for it, appalled at the blood ritual. She did this to save her husbands life, not as an act of submissive obedience to God. As always there is more to the Scripture than the obvious, literally it says "a blood-bridegroom art thou to me," addressed to Moses, (Keil & Delitzsch Commentary on the Old Testament). The skin was cast at Moses feet, the one who would eventually give the law to Israel.

Jesus was a husband of blood to his bride. The son, (children) are spiritually circumcised from the world so they can serve God. Without this done Moses could not serve God even though he was chosen for this task. God asks obedience in what he has already commanded before we are sent with his blessing to new step of faith.