Did you ever wonder why God chose circumcision as the sign of the Abrahamaic covenant? It is a rather odd symbol. Saint Thomas Aquinas explains why God chose the removal of the male foreskin as the sign of faith.
Nevertheless in the Old Law the remedy against original sin was affixed to the member of procreation; because He through Whom original sin was to be removed, was yet to be born of the seed of Abraham, whose faith was signified by circumcision according to Romans 4:11 (Summa theologiae III, q. 66, a. 7, ad 3).