Here’s another commentary I read by James Kugel in his book “How to Read the Bible”.
It seems inexplicable that God tries to kill Moses and Zipporah’s response of circumcising her son and touching Moses’ feet with the bloody foreskin.
Ancient interpreters assumed that since God stops trying to kill Moses after the circumcision, Moses’ sin must have been in delaying the circumcision of his son.
“Great indeed is the commandment of circumcision, for not the slightest delay was granted even to the righteous Moses in regard to it.” - Mishna, Nedarim 3:11
This was a relevant lesson, since Jews had been pressured, at various times, to abandon circumcision.
But why would Moses delay? Interperters reasoned that since Moses was in Midian and his father in law was a pagan priest, he was unable to circumcise his child until after he left Midian. ANd since Moses did not immediately circumcise his son (or sons) after leaving Midian, God tried to kill him.
Here’s what modern scholars think:
The key to this episode lies in the Hebrew word “az”. Literally, as mean “then”, but it is often used to convey “that was when”. When used as “that was when” it implies that the reader has already heard something about the matter being discussed. So here, the narrative says “That was when [the expression] ‘bridegroom of blood’ was first used in connection with circumcision.”
So the whole story is to explain the relationship between circumcision and the term ‘bridegroom of blood.’ But why was this important?
Answer: Because the Torah insists that circumcision be performed on infants. This was specifically an Israelite practice. Many other cultures circumcise as a rite of puberty or prior to marriage. Apparently “bloody bridegroom” was a term in use at the time the story of Moses was written. So the Torah is trying to show that bridegroom of blood does not mean circumcision in adulthood, but is rather tied to the Israelite concept of circumcision. So the expression does not mean (and never did) that future bridegrooms should be circumcised at puberty or a few months before the wedding.
So, to summarize: The entire purpose of the story, according to this theory, is to explain that the phrase “bridegroom of blood” was coined to refer to Moses, Zipporah’s bridegroom, when she touched him with the bloody foreskin of their newborn son and so saved his life.