That is, they have already received that inward circumcision, that spiritual cleansing of the heart, that is effected by the Holy Spirit. In either case, another contrast with circumcision is in view. Physical circumcision removes a small piece of flesh. Paul's opponents might well have agreed that an inward cleansing was in order. However, they would have insisted that this be signified by physical circumcision.
Physical circumcision has nothing to add.
The Abrahamic covenant is fulfilled in the new covenant, and circumcision has been replaced by baptism as the sign and seal of the righteousness of faith. Children in the Church Under the Abrahamic covenant, those who were born within the covenant community received the sign of the covenant as infants. Because the Abrahamic covenant remains in effect, but with the sign of it having been changed, it follows that those who are born within the covenant community should be baptized as infants, just as they were formerly circumcised as infants. There is no lesson for adults to learn unless Jesus welcomes the infants of believers into his kingdom. Since we today are part of that covenant through faith in Christ, the new sign of the covenant, water baptism, should likewise be applied to infants on the basis of parental faith.