Ambrose of Milan — on 2Cor 3:6 (Letter 74, To Irenaeus)

“Rightly therefore does Paul say that the letter killeth but the spirit giveth life. For the letter circumcises a small portion of the body; the understanding spirit keeps the circumcision of the whole soul and body; that the superfluous parts being cut off, (for nothing is so superfluous as the vices of avarice, the sins of lust, which nature had not, but sin caused,) chastity might be observed, and frugality loved. The sign therefore is bodily circumcision, but the truth is spiritual circumcision, the one cuts off the member, the other cuts off sin. Nature has created nothing imperfect in man, nor has she commanded it to be taken away as if it were superfluous, but that they who cut off a part of their body might perceive that sins were much more to be cut off, and those members which led to offences were to be retrenched, even though they were joined together by a certain unity of body, as it is written, "If thy right hand offend thee, cut it off, and cast it from thee, for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell."”