For circumcision expressed nothing else, but the stripping off of the old birth, seeing that part was circumcised which caused the birth of the body. And thus it was done at that time as a sign of the future baptism through Christ. Therefore as soon as that of which it was a sign came, the figure ceased. For since the whole of the old man Adam is taken away by baptism, there remains nothing which the cutting of a part prefigures.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: De Sabbato et Circumcisione.