Here’s an enigmatic story! After the Lord has patiently heard Moses’ objections and overcome them, after Moses agrees to do what he is still reluctant to do, after he gets permission from his Father-in-law and readily obtains it, and after he leaves Moab to return to Egypt the Lord comes to him at night and tries to kill him.
How? What must it have been like to be attacked unexpectedly at the encampment? Not much information is given. The question ‘how’ goes unanswered because it is not our business.
The question instead is, Why. Why did the Lord meet Moses and then try to kill him? Why here, why now?
The answer has to do with the past and the future. The Lord’s meeting with Moses takes place between the past and the future, in the present, where all his encounters with temporal beings occur.
What is in the past? In the distant past, a covenant. A covenant sealed by circumcision. A covenant of dedication to God, of hoping in him exclusively, of waiting and trusting in his promises, of offering him a whole entire life. In the near past, are the words of God spoken in Moab before Moses set out. Do all the wonders, God says in 4:21; Pharaoh is not going to listen to you; then you are going to tell him something about Israel whom I mean to deliver: tell him that it is my firstborn. And because you will not let my firstborn go and serve me, I will kill your own firstborn.
That is in the past, and one other thing. In the present Zipporah realizes what God is after and abruptly circumcises her son, bitterly exclaiming that Moses is a husband of blood. She knew! She knew what God was after. And the question is how she would have known. How did Zipporah find out in the confusion of that desperate moment that was what God wanted?
Because in the past she must have objected to Moses’ Hebrew rather than Egyptian identity. And we must assume that she did not want the reproach of God’s enslaved people. She was disgusted at it and by it. And Moses must have listened to his wife, the way Adam did, the way Abraham did, forgetting the past, the covenant, God’s righteous demands.
But the future needs to factor into understanding this enigmatic episode as well. The future is what God plans to do: to rescue his consecrated people, those who have a covenant relationship with him, the circumcised children of Israel. And the Lord’s purposes for his people involve the punishment of Pharaoh. The climactic plague is the one that sets up the Passover, that festival of identity which Christians still maintain in the Lord’s Supper. And it is the one in which God purposed to kill Pharaoh’s firstborn, his best, and the best and first of all the land, to show him the enormity of what he is doing by enslaving God’s own firstborn and keeping him from service to God.
And so Moses’ own firstborn must identify with God’s covenant and God’s people, must be circumcised, must be consecrated with blood to the Lord. “You are a husband of blood!” Zipporah exclaims, because she knows God’s requirements, though she does not understand their saving, precious meaning. Moses could not be about God’s business without acknowledging the rituals God intended as vehicles toward that precious, all-important meaning: the great and ultimate meaning which is found in Christ, in his blood, in his consecration, in our union to him by faith, and in the circumcision of our hearts which sets them apart for his service.