Genesis 17:10

Gen. 17:10. Circumcision signified or represented that mortification or the denying of our lusts, that is the condition of obtaining the blessings of the covenant. Totally denying any lust, is represented in Scripture by cutting off. Thus, cutting off a right hand, or right foot, is put for the denying of some very dear lust; so cutting off the flesh of a member so prone to violent lust, signifies a total denying of our lusts. A main reason why lust, or our natural corruption, is represented by the instrument of generation, is because we have all our natural corruption or lust by generation, i.e. by being the natural offspring of the corrupt parents of mankind. Therefore when God would signify that our original or natural corruption should be mortified, he appoints that the flesh of the part specified should be cut off.

Another reason why the seal of the covenant that God made with Abraham was appointed to be affixed to this part of the body, seems to be that God made this covenant not only with Abraham and for him, but him and his seed. It mainly respected his seed, as abundantly appears by the tenor of the words, in which the covenant was revealed from time to time; and therefore the seal was to be affixed to that part of the body whence came his seed. The covenant was made not with a man, but with a race of men ordinarily to be continued by natural generation; and therefore the sign of the covenant was a sign affixed to the instrument of generation. The sign was a purgation of the member of the body, by which offspring was procured, and was to be a sign of the purification of the offspring. God seeks a godly seed, and children that are holy.

Corol. Hence we learn that seeing the Gentiles now in the days of the gospel are admitted to the seal of Abraham as the Jews were, and are admitted to an interest in Abraham's covenant, and to the blessing of Abraham, so that Abraham is become the father now, not of one nation, but of many nations in the way of that covenant, as the apostle Paul abundantly teaches; then the posterity of Christians by natural generation are now God's people, and are a holy seed by Abraham's covenant, as the Israelites were of old. There are but two ways in which persons can become of Abraham's covenant, race, or generation: one is by generation by the natural instruments of generation, to which the seal of the covenant was affixed, and so continued form the root to the branches; the other is by ingrafting a new branch into that stock, that shall after ingrafting grow and bring forth branches, and bear fruit upon that stock, as the other branches did that were cut off to make room for them. In this way now many nations or generations are of Abraham's race, instead of one nation or family.

Gen. 17:12