If any one in sickness has been subjected by physicians to a surgical operation, or if he has been castrated by barbarians, let him remain among the clergy; but, if any one in sound health has castrated himself, it behoves that such an one, if [already] enrolled among the clergy, should cease [from his ministry], and that from henceforth no such person should be promoted. But, as it is evident that this is said of those who wilfully do the thing and presume to castrate themselves, so if any have been made eunuchs by barbarians, or by their masters, and should otherwise be found worthy, such men the Canon admits to the clergy.
Ancient Epitome1 Of Canon I.
Eunuchs may be received into the number of the clergy, but those who castrate themselves shall not be received.
The divine Apostolic Canons xxi., xxii., xxiii., and xxiv., have taught us sufficiently what ought to be done with those who castrate themselves, this canon provides as to what is to be done to these as well as to those who deliver themselves over to others to be emasculated by them, viz., that they are not to be admitted among the clergy nor advanced to the priesthood.
(Smith & Cheetham, Dict. Christ. Ant.)
The feeling that one devoted to the sacred ministry should be unmutilated was strong in the Ancient Church .... This canon of Nice, and those in the Apostolic Canons and a later one in the Second Council of Arles (canon vii.) were aimed against that perverted notion of piety, originating in the misinterpretation of our Lord's saying (Matt. xix. 12) by which Origen, among others, was misled, and their observance was so carefully enforced in later times that not more than one or two instances of the practice which they condemn are noticed by the historian. The case was different if a man was born an eunuch or had suffered mutilation at the hands of persecutors; an instance of the former, Dorotheus, presbyter of Antioch, is mentioned by Eusebius (H. E. vii., c. 32); of the latter, Tigris, presbyter of Constantinople, is referred to both by Socrates (H. E. vi. 16) and Sozomen (H. E. vi. 24) as the victim of a barbarian master.
We know, by the first apology of St. Justin (Apol. c. 29) that a century before Origen, a young man had desired to be mutilated by physicians, for the purpose of completely refuting the charge of vice which the heathen brought against the worship of Christians. St. Justin neither praises nor blames this young man: he only relates that he could not obtain the permission of the civil authorities for his project, that he renounced his intention, but nevertheless remained virgo all his life. It is very probable that the Council of Nicea was induced by some fresh similar cases to renew the old injunctions; it was perhaps the Arian bishop, Leontius, who was the principal cause of it.2
Constantine forbade by a law the practice condemned in this canon. "If anyone shall anywhere in the Roman Empire after this decree make eunuchs, he shall be punished with death. If the owner of the place where the deed was perpetrated was aware of it and hid the fact, his goods shall be confiscated." (Const. M. 0pera. Migne Patrol. vol. viii., 396.)
The Nicene fathers in this canon make no new enactment but only confirm by the authority of an Ecumenical synod the Apostolic Canons, and this is evident from the wording of this canon. For there can be no doubt that they had in mind some earlier canon when they said, "such men the canon admits to the clergy." Not, ou[toj o9 kanw\n, but o9 kanw\n, as if they had said "the formerly set forth and well-known canon" admits such to the clergy. But no other canon then existed in which this provision occurred except apostolical canon xxi. which therefore we are of opinion is here cited. [In this conclusion Hefele also agrees.]
This law was frequently enacted by subsequent synods and is inserted in the Corpus Juris Canonici, Decretum Gratiani. Pars. I. Distinctio LV., C vij.