Charles Ellicott Commentary Genesis 17:13

"He that is born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant." — Genesis 17:13 (ASV)

He that is born in your house ... — Two things follow from this wide extension of the rite of circumcision. The first is that all members of Abram’s household, being thus sharers in the covenant, were also numbered as belonging to the nations that sprang from him. We have seen that even in early days his followers must have numbered six or seven hundred men (Genesis 14:14), and they were growing in multitude throughout the rest of his life and during the lifetime of Isaac.

They were then divided between Esau and Jacob at Isaac’s death (Genesis 35:27; Genesis 36:6–7), but the diminution in the number of Jacob’s family thus caused must have been compensated by those whom he gathered for himself in Mesopotamia (Genesis 30:43). All his household went down with him into Egypt as part of his taf—translated little ones (Genesis 46:5)—which really signified the whole body of dependents: men, women, and children.

Placed there in the fruitful Delta, they would be counted as members of the tribe to whose chief they belonged, and they would swell the numbers of the vast host that left Egypt (Exodus 12:37). The second point is that, just as all who were circumcised were regarded as Israelites, so also circumcision was confined to the Israelites. It was not a catholic ordinance, intended, like baptism, for all people and all times. Nor was it primarily a religious institution. The bought slave was circumcised first and instructed afterwards.

No profession of faith was required; instead, he was admitted to the privilege by right of his master. The reason for this was that it was an admission into the Jewish nation first, and only consequently into the church. This is one of the many points that distinguish slavery as practiced among the Jews from the degrading form it took in modern times: from the days of Abram onwards, the slave, by being circumcised, was proclaimed to be one of the same race and nation as his master and was thereby entitled to share in his national and religious privileges.