Commento su Genesi 17:13

המול ימול יליד ביתך HE THAT IS BORN IN THY HOUSE MUST NEEDS BE CIRCUMCISED — Here the statement that one born in the house must be circumcised is repeated but it does not say “after eight days” as in 5:12, thus teaching you that there are some cases of those “born in the house” who have to be circumcised on the first day of birth, as is explained in Treatise Sabbath (Shabbat 135b).

בבשרכם; the word בשר is a euphemism for the reproductive organ of the male, as for instance in Ezekiel 16,26 גדלי בשר, "with outsized male member." Or, Leviticus 5, 2-3 זב מבשר, "an uncontrolled seminal emission from his male organ." There are more examples of the word בשר having that same meaning. Seeing that the organ on which the covenant with G'd is marked is the organ instrumental in ensuring a man's continuity beyond death through the genes of his offspring, it is appropriate that the covenant which represents the eternal intimate and reciprocal relationship of G'd and the Jewish people should be symbolized in this manner.

המול ימול, this refers to Avraham ensuring that all the male members of his household be circumcised now with him. The Torah had already issued instructions in the previous verse that Avraham's male descendants were to be circumcised, as well as any gentile bodily owned members of their households. Just as they were to study G'd's laws, i.e. "His ways," they would have to carry out this commandment as it is inconceivable for more than one religion to be observed within the same household. Neither were there to be members of two different nations within the same household, one of whom circumcised and the other uncircumcised.