This He said not because proselytes were circumcised, but because they imitated the lives of those from following whom He bad prohibited His disciples, saying, “Do ye not after their works.” [Matt 23:3]
Two things are observable in this command; first, the honour shewn to Moses’ teaching, that even wicked men when sitting in his seat are compelled to teach good things; and that the proselyte is made a child of hell, not by bearing the words of the Law, but by following their doings. And twofold more than they for this reason, that he neglects to fulfil what he had undertaken of his own choice, having been not born a Jew, but of free will become a Jew.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: cont. Faust., xvi, 29 (et cf cont. Adimant. 16)