John Chrysostom, on Gen 17:14

See the Lord's wisdom in knowing how inobservant future generations are likely to be, and so, as though putting a bit in their mouths, he gave them this sign of circumcision, curbing their unrestrained urges in case they should mingle with other peoples. You see, he was aware of their lustful tendencies in not practicing restraint, even though it had been drummed into them countless times to refrain from their irrational impulses. Consequently he gave them a perpetual reminder with this sign of circumcision, as though fastening them with a chain. He set limits and rules to prevent them overstepping the mark instead of staying within their own people and having no association with those other peoples but rather keeping the patriarch's line uncontaminated. In this way even the fulfillment of the promises could be achieved for their benefit. It is like a man of self-control and good sense having a disobedient child; he puts limits and rules on him not to show his face outside the front door or to be seen by passersby. In fact, he oftentimes ties him up by the feet so as to succeed in this way in getting the better of his extreme indiscipline. Well, in just the same way the loving Lord also placed this sign of circumcision in their flesh, like shackles on their feet, so that with this reminder at home they might have no further need of instruction from others.