Roman Emperor Hadrian issued a decree outlawing circumcision around 132 CE, under penalty of death. I am curious as to how such a decree may be enforced, given that circumcision/castration practices are quite private.
There is quite a problem in concluding that that decree even existed in the first place, as it is now interpreted, whether it is really about circumcision or castration. If it existed, who decreed it, and what exactly it contained or how it was to be applied is equally debated. The available evidence from Roman sources is extraordinary weak and most works for the last 1900 years drew heavily on mainly Jewish or Christian sources written long after the event. That has only changed very recently, and not without objections, when scholars went again ad fontes.
Whether it was more a symbolic policy, weakly enforced, or indeed a failed attempt to repeat the equally failed and much harsher implemented Seleucid goals remians unclear.
In any case, circumcision was a somewhat public ritual, to be celebrated openly, not a quick dodgy procedure, done in a shack far from anyones eyes.
"The Circumcision" by Vincenzo Catena, used in:
Andrew Jacobs: "On the Eighth Day of Christmas…", Ancient Jew Review, 2016.
Most problematic afterwards was the 'public inspection' that would routinely happen in the hellenised, then romanised East: the shameful nakedness in gymnasiums or baths. Quite a few adult males went to lengths to restore what was cut to hide the fact that something was missing –– and they were different.
So there were quite some voluntary "body checks" for those males that wanted to participate. Another route of exposure would of course be accusations and denunciation, like with many Christians.
As Antoninus Pius gave just a few years later explicit permission for Jews to practice this body modification (again, and this time only on Jews, not their 'property', that is slaves, ending proselytism) it indeed does not stand to reason that it was even really banned beforehand, merely that it was a contested form of genital mutilation that some people objected to and others would really like to perform. [This is not a comment on current "barbarism against boys", but a translation from the Latin mutilare genitalia.] Furthermore, the Romans really had an aversion to the practice, as did the Greeks, but were quite tolerant in Egypt, repealing it quite quickly, where the practice was also widespread, especially among priests.
For the Jews and for the Egyptians, for whom circumcision was a ritual of vital importance, Hadrian's edict was a disaster. Protests soon arose and a dual exception had to be introduced into the new regulation. The Egyptians, cherished by Hadrian, were the first to obtain satisfaction: the privilege permitting the circumcision of Egyptian priests was promulgated immediately after the edict. The Jews had to wait longer. It is possible to establish a chronology of this legislation, concerning the edict itself as well as the dual exception in favor of the Egyptians and of the Jews. To this effect; we must compare the juridical texts which we just examined with documentary evidence contained in the Greek papyri from Egypt.
That the emperor Hadrian issued a general prohibition of circumcision is beyond dispute, but when and why he did so has been debated. Now, by connecting Greek papyri from Egypt with Roman juristic writings we are able to determine the date and the form of Hadrian's ban on circumcision: an imperial edict to be dated 119/120 CEo The ban provoked conflicts between the Roman government and nations attached to the ritual of circumcision, the Egyptians and the Jews. We have seen how the conflict was settled as far as the Egyptians were concerned. How did the Jews fare?
To bring this to a definitive end, Emperor Hadrian decided to forbid surgery on genital organs in any form whatsoever. The widespread confusion equating circumcision with castration facilitated his decision.
The mutilatio was illegal only when it was perpetrated for commercial purposes or for the satisfaction of "loose desires," libidinis vel promercii causa; thus, circumcision remained permissible when it was effectuated with a religious aim, provided that the permission had been clearly formulated as an official norm. Such was not the case of the Syrian Elagabalus who circumcised himself in honor of his Syrian god when he became Roman emperor; he did not, however, consider himself bound by a strict observance of the law: principi omnia licent. The only legal exceptions formally in force were those we already know concerning the Egyptian priests and the Jews. And these were rigorously restrictive.
Joseph Meleze Modrzejewski: "'Filios suos tantum' - Roman Law and Jewish Identity", p108–136, in: Menachem Mor, Aharon Oppenheimer (Eds): "Jews and Gentiles in the Holy Land in the Days of the Second Temple, the Mishnah and the Talmud", Yad Ben-Zvi Press: Jerusalem, 2003.
Given the above, newer evidence on "authorship", the following reads with quite some caveats. I'd advise against reading either the above or the below as "the final word on it":
Historia Augusta, Vita Hadriani, 14.2:
Moverunt ea tempestate et Iudaei bellum, quod vettabantur mutilare genitalia.
This single line in the biography of Hadrian, from a collection of biographies of the Caesars called Historia Augusta, has been interpreted as evidence that Hadrian forbade the Jews to perform circumcisions.
David Golan referred also to the grotesque and ironic style of writing in Historia Augusta. In his view, circumcision is mentioned in a context of mockery for the sake of thaumasia (an extraordinary kind of the strange-grotesque that also arouses amazement). In his words, “No less telling is the fact that the editor-author rather than choose the prevailing technical or juridical term for describing circumcision in his text, that is circumcidere, preferred an expression which bluntly associated it with castration. The literary effect to which the writer aimed seems obvious, combining erotic innuendos and scoffing remarks on account of the oddity of the Iudaei.”
Peter Schäfer also rejected the circumcision decree as a cause for the Second Revolt. In his view, this prohibition was imposed during the course of the rebellion or after its suppression, and the issue concerning circumcision was part of an internal Jewish struggle without any connection to Hadrian the Emperor.
Yossi Geiger gave a different interpretation of the sources by also detaching the circumcision prohibition from the name of the emperor. He cast doubt on the reliability of the evidence given by Spartianus and also rejected the indirect evidence in the Digest of Modestinus on permission to circumcise, which Antoninus Pius gave the Jews. In the view of Geiger, the circumcision prohibition was made at the initiative of the governor of Judaea, Tinius Rufus, who decreed it in the Coercitio following the uprising of the Jews when Aelia Capitolina was being established.
Menahem Mor: "The Second Jewish Revolt: The Bar Kokhba War 132-136 CE", (Brill Reference Library of Judaism), Brill: Leiden, Bosten, 2016.
The great Mary Edith Smallwood asked already in 1959:
Or, to ask the same question in different words, was the prohibition the cause or the consequence of the revolt of the Palestinian Jews in 132-5 ? For that revolt can hardly have been unconnected with a measure which, if enforced, would have struck a death-blow to Judaism.
E Mary Smallwood: "The Legislation of Hadrian and Antoninus Pius against Circumcision", Latomus, T. 18, Fasc. 2 (AVRIL-JUIN 1959), pp. 334-347.
So, while the practice was outlawed and frowned upon on several levels, and had quite deep consequences for the parting of the ways between Pauline Christianity and emerging Judaism, enforcement of these pagan norms was not as strict as anyone imagining an efficient Roman bureaucracy in the time of Hadrian envisions:
The one with a drawn down [foreskin] must undergo circumcision. R. Judah says: He is not required to undergo circumcision if he drew down, because this entails danger. They said: Many underwent circumcision in the time of Bar Kozeva (Bar Kokhba), had children, and did not die.
What emerges from this source is confirmation that in the period of the Bar Kochba revolt there were individuals who had undergone decircumcision and who might have been subject to the death penalty if they chose to renew their circumcisions. The Rabbis obligated them nevertheless to undergo a second circumcision, despite the danger this posed. R. Judah alone exempted them, but the text seems to imply that were it not for the death penalty he too would have required a mashukhto undergo a second circumcision.
Nissan Rubin: "Brit Milah: A Study Of Change In Custom", p87–97, in: Elizabeth Wyner Mark (Ed): "The Covenant of Circumcision: New Perspectives on an Ancient Jewish Rite", Brandeis University Press: Waltham, 2003. (online).
I can't speak to that decree specifically, but Roman bureaucracy was, by modern standards, ludicrously small and ineffective. There's a very good chance that it was enforced (or not) according to the whims of the local town government.
We have correspondence between Pliny the Younger (then governor of Pontus/Bithynia) and Trajan (Hadrian's immediate predecessor) wherein Pliny asks Trajan how to deal with Christians given that it being one was punishable by death. Trajan replied,
You observed proper procedure, my dear Pliny, in sifting the cases of those who had been denounced to you as Christians. For it is not possible to lay down any general rule to serve as a kind of fixed standard. They are not to be sought out; if they are denounced and proved guilty, they are to be punished, with this reservation, that whoever denies that he is a Christian and really proves it--that is, by worshiping our gods--even though he was under suspicion in the past, shall obtain pardon through repentance. But anonymously posted accusations ought to have no place in any prosecution. For this is both a dangerous kind of precedent and out of keeping with the spirit of our age.
Two points to note: Trajan didn't really want to bother with this "they are not to be sought out" and if Pliny was forced to deal with them, let them off -- no matter what the evidence -- if they said "Sorry" and sacrificed.
Secondly, Pliny had to write to Trajan. The Empire (then at its peak) didn't have established policies for dealing with even capital crimes and neither Pliny nor Trajan had a large bureaucracy to deal with it. (It's quite probable that the letters were written by staff, but it's likely that Pliny and Trajan were directly involved in the matter.)
Once whatever moral panic that set off the anti-circumcision edict was over, it was probably put into the "Oh, please, I hope no one remembers that this is on the books" category by the various Roman authorities.
Suetonius gives a firsthand account of how an old man was stripped naked in public in order to check whether he was circumcised.
I remember, when I was a youth, to have been present, when an old man, ninety years of age, had his person exposed to [view] in a very crowded court, in order that, on inspection, the procurator might satisfy himself whether he was circumcised. (Life of Domitian, 12)
This doesn't have to do with the specific case of Hadrian's decree, but with the Jewish tax levied the previous century. However, it does show how the Romans were able to enforce circumcision laws.