Now the reason why circumcision is practised among the Jews is not the same as that which explains its existence among the Egyptians and Colchians, and therefore it is not to be considered the same circumcision. And as he who sacrifices does not sacrifice to the same god, although he appears to perform the rite of sacrifice in a similar manner, and he who offers up prayer does not pray to the same divinity, although he asks the same things in his supplication; so, in the same way, if one performs the rite of circumcision, it by no means follows that it is not a different act from the circumcision performed upon another.
For the purpose, and the law, and the wish of him who performs the rite, place the act in a different category. But that the whole subject may be still better understood, we have to remark that the term for “righteousness” is the same among all the Greeks; but righteousness is shown to be one thing according to the view of Epicurus; and another according to the Stoics, who deny the threefold division of the soul; and a different thing again according to the followers of Plato, who hold that righteousness is the proper business of the parts of the soul. And so also the “courage” of Epicures is one thing, who would undergo some labours in order to escape from a greater number; and a different thing that of the philosopher of the Porch, who would choose all virtue for its own sake; and a different thing still that of Plato, who maintains that virtue itself is the act of the irascible part of the soul, and who assigns to it a place about the breast. And so circumcision will be a different thing according to the varying opinions of those who undergo it.
But on such a subject it is unnecessary to speak on this occasion in a treatise like the present; for whoever desires to see what led us to the subject, can read what we have said upon it in the Epistle of Paul to the Romans.
Source: Against Celsus (New Advent)