The pronouns in the "Bridegroom of Blood" story are unclear, both in translation and in the original Hebrew. The word "he" gets repeated multiple times with no clarification about whether it's referring to Moses, Moses's son, or even God. In fact, the whole story is unclear and confusing. What seems to be going on is that God attacked Moses, and Tzipporah touched the foreskin of her son to Moses’s legs, but “his legs” could refer to her son or to God Himself.
One partial explanation for what’s going on here comes from the commentator Rashi, who says “because he [Moses] had not circumcised his son Eliezer; and because he had showed himself remiss in this, he brought upon himself the punishment of death.” Circumcision was, and is, a very big deal. In this scene, Moses is on his way to Egypt to speak to Pharaoh in order to free his fellow Israelites. It would be hypocritical if he claimed to speak for his people while refusing to uphold one of his people’s most important traditions.
Translator Everett Fox takes a more literary approach, saying this scene “serves as an end bracket to Moses’s sojourn in Midian. As mentioned earlier, Moses flees Egypt under pain of death; here, on his return, he is in mortal danger once more. Second, our passage seems to be an inclusio or bracketing passage for the entire Plague Narrative… God, designating Israel his firstborn and alluding to the future killing of Pharaoh’s/Egypt’s firstborn sons, demonstrates his power as a life-taker, to be pacified or turned away only be a ceremonial blood-smearing - parallel to the Israelites’ smearing of blood on their doorposts when their own firstborn are threatened by the Tenth Plague.
Two final points should be noted here. First, it is with the act of his son’s circumcision that Moses finally becomes a true Israelite… And second, it is telling, again, that the person who saves Moses’s life in adulthood is a woman. In a sense, Moses’s early life is now over, having come full circle.”
Both of these are good interpretations, but here’s my take: If the issue was that Moses had not fulfilled the obligation of circumcision, then Moses should have been the one to circumcise his son. But Tzipporah is the one who does it. That implies that it was Tzipporah who needed to prove something. What was she proving? Well, she seems to be proving her child’s right, and by extension her own right, to be included in God’s covenant. She is physically marking her son as an Israelite.
Why does this scene happen now? She and Moses have been married for some time. If the issue was that she and her child were not “really” part of God’s people, you’d think God would have objected much sooner. So why does God choose this moment to intervene?
Tzipporah is a Midianite. She has no personal experience of slavery and subjugation. Here, she gets a taste of what the Israelites have suffered. Pharaoh brought death upon the male infants; God threatens death upon Tzipporah’s male family members.
“This is what it’s like,” God seems to be saying, “to be helpless before someone stronger than you. This is what it’s like to see those you love in danger. This is what your husband’s people - your people, if you choose to claim them - have suffered for four hundred years. Can you handle it?”
Tzipporah, unequivocally, answers yes. Yes, she’s prepared to do whatever is necessary to protect her family. Yes, she understands the obligation she’s taken on. Yes, she’s prepared to uphold God’s commandments, even the difficult ones. It’s a moment of conversion. A bloody, painful conversion, but a conversion nonetheless.
The Israelite people are often referred to as the bride of God. Rashi, commenting on Numbers 7:1, says “on the day that the Tabernacle was erected Israel was like a bride”; similarly, his commentary for Exodus 19:17 states “the Shechina [another name/aspect of God] was going forth to meet them, as a bridegroom who goes forth to meet his bride.” In this context, the phrase “a bridegroom of blood” makes more sense. Tzipporah is addressing God directly, just as her husband does. She treats the circumcision as a marriage vow binding her and God together. She is ready to take her place as part of the Israelite people.