When Paul urges the Judaizers who trouble the Galatian churches to “mutilate themselves” (Galatians 5:12), it seems like a crude joke. They like cutting foreskins; might as well go all the way to cut off the whole thing.
But it’s not just a vulgar insult. As many commentators have noted, Paul virtually equates circumcision with castration, which, far from being a mark of inclusion in Israel is a reason for exclusion (Deuteronomy 23:1). But a student, Grace Langness, points out that there may be something else happening. Circumcision is literally a removal; it is a privation. To take pride in circumcision is to take pride in an absence. This is the irony of the Judaizers position: They treat circumcision as a sign of power, when in fact it is a sign of human impotence, the impotence of the flesh. Whey they glory is circumcision, they glory in what is not there; they glory in nothing.
Castration is not, in this sense, the opposite of circumcision, but the logical extension of circumcision. If you want to glory in nothing, you might as well not stop cutting when you’ve cut the foreskin. Go all the way and boast in absolutely nothing. If the Judaizers would do that, though, they would circle back to Paul’s own position, because they would be glorying in their own powerlessness, the inability of flesh to breed Spirit, the incapacity of humans do rectify what has gone bad. If they would castrate themselves, they would begin to see the truth of circumcision: That it is nothing.