And he that is eight days old shall be circumcised among you, every male child in your generations, he that is born in the house, or bought with money of any foreigner, who is not of your descendants.
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Ambrose of Milan
And because he is called to what is perfect, Abraham receives the oracle that summons to perfection. “Circumcise,” it says, “every male of yours and circumcise your flesh”; but perfect circumcision is the spiritual one. Indeed, Scripture also teaches this when it says, “Circumcise the hardness of your heart.” Even here many interpret the text to be saying, “Circumcise every male of yours,” that is, your minds; for nothing is more virile than the mind. Moreover, because the male is also holy, it is said, “Every male opening the womb shall be called holy for the Lord.” But what is holier than the mind that produces the seeds of good thoughts by which it opens the womb of the soul, which was closed by the sterility that prevented it from childbearing, so that it might give birth to invisible generations, obviously through that spiritual womb of which Isaiah said, “We have conceived in the womb and given birth to the spirit of salvation”? So what is mandated is the intelligible circumcisio...
Days, when he will be able to bear the pain without danger. This might be deferred for a just reason, as it was in the desert, Josue v. 6. In this case people might be saved, as younger children and all females might, by the application of the remedies used in the law of nature, sacrifice, the faith of parents (Menochius)
Of your stock, and, being arrived at years of discretion, is desirous of enjoying your privileges. Some think, that slaves had no choice left; but servants, and people who had a mind to live in the country, were not bound to submit to this rite against their will. It is even more probable, that none were under this obligation, except Abraham and his posterity by Isaac. His other children adopted it in part, but not with the exactitude of the Jews. (Calmet) ...