Evidently God waited a while (13 years actually) after Abraham's foolishness with Sarah's young Egyptian servant before He approached him again. And, when He came to Abraham again, he came instructing him thusly, "Walk before me, and be thou perfect." Apt injunction, eh?
At this point, God did several things in establishing His covenant with Abraham. First, He changed his name and his wife's name. Abram (exalted father) became Abraham (father of a multitude), and Sarai (princess) became Sarah (noblewoman). Of course, God changes us all when we enter into a covenant relationship with Him. However, the name change wasn't the most drastic adjustment on the docket. The second change was physical (for the males) and deeply representative. Circumcision! The word circumcise means "to cut around" . . . enough said?
Why would God pick this particular rite of passage to symbolize the agreement He was making with Abraham and his offspring? Well, I don't know exactly what He had in mind, but it is fairly obvious to me that God picked something incredibly intimate because He desired an incredibly intimate friendship with His creations. Not only that, as revelation progressed, God expressed the more pertinent need for men and women both to have their hearts circumcised. While physical hygiene, sensitivity and aesthetics had some temporal value; spiritual hygiene, sensitivity and aesthetics have eternal value. Remember, ''Man looks on the outward appearance, but God looks on the heart" (1Samuel 16:7). Of course, Abraham's heart had ALREADY been surgically adjusted prior to any change in his anatomy.
One might ask, "If God had Abraham's heart, why did He require his foreskin too?" Well, I think James explains this clearly enough: "Faith without works is dead" (James 2:20b). God seems to always give us some visible way of identifying with invisible realities; skin coats for Adam and Eve, a rainbow for Noah, circumcision for the Jews… and baptism for us.