I was just reading some history on extreme forms of asceticism and monasticism. In passing the subject of Origen it said he had castrated himself. Is this generally considered true by church historians?
This has been debated, so no one knows for sure. The consensus of the scholars seems to be that this was a malicious rumor about him, possibly started by Demetrios, the Bishop of Alexandria, a prosecutor of Origen.
John McGuckin's Westminster Handbook to Origen states that someone who interpreted the Gospels so allegorically would be unlikely to have interpreted Matthew 19:12 so literally and that in Origen's commentary on Matthew, he himself derides the literal interpretation of the eunuch and says only an idiot would consider that. Origen doesn't use the word "idiot," but you can read his derision for the interpretation in his Commentary on Matthew 15.1-5.
The castration story comes from the historian Eusebius of Caesarea. Eusebius, the church historian who lived a generation after Origen, devotes nearly all of Book VI of his Ecclesiastical History to the life of Origen. While some scholars have cast doubt on the reliability of this source due to the author's obvious admiration for his subject and the work's loving hagiographical tone, most do not believe these factors are reason for doubting Eusebius' reports. However, why take Eusebius' account over Origen's own words?
A nice account can be found here: http://houseoftheinklings.blogspot.com/2006/09/on-origen.html
Many historians argue that Eusebius, who lived only a few decades after Origen, would have access to accurate information and no incentive to lie. Others argue that much of what Eusebius says cannot be considered historically reliable, pointing out that Origen's self-castration would conflict with Origen's theology: he interpreted Mat 18:8-9 as a command to separate from people who are causing one to sin, not a command to rid oneself of body parts. Thus it is very unlikely that he took Matthew 19:12 to be about amputation.
ORIGEN ON MATTHEW 18:8-9: "If your hand or foot causes you to sin, cut it off and cast it from you. It is better for you to enter into life lame or maimed, rather than having two hands or two feet, to be cast into the everlasting fire. And if your eye causes you to sin, pluck it out and cast it from you. It is better for you to enter into life with one eye, rather than having two eyes, to be cast into hell fire."
And it is well, then, if the eye and the hand are deserving of praise, that the eye cannot with reason say to the hand, I have no need of you. 1 Corinthians 12:21 But if any one in the whole body of the congregations of the church, who because of his practical gifts has the name of hand, should change and become a hand causing to stumble, let the eye say to such a hand, I have no need of you, and, saying it, let him cut it off and cast it from him. Matthew 18:8 And so it is well, if any head be blessed, and the feet worthy of the blessed head, so that the head observing the things which are becoming to itself, may not be able to say to the feet, I have no need of you. If, however, any foot be found to become a stumbling-block to the whole body, let the head say to such a foot, I have no need of you, and having cast it off, let him cast it from himself; for even it is much better that the rest of the body should enter into life, wanting the foot or the hand which caused the stumbling-block, rather than, when the stumbling-block has spread over the whole body, it should be cast into the hell of fire with the two feet or the two hands. And so it is well, that he who can become the eye of the whole body should be worthy of Christ and of the whole body; but if such an eye should ever change, and become a stumbling-block to the whole body, it is well to take it out and cast it outside the whole body, and that the rest of the body without that eye should be saved, rather than that along with it, when the whole body has been corrupted, the whole body should be cast into the hell of fire. For the practical faculty of the soul, if prone to sin, and the walking faculty of the soul, so to speak, if prone to sin, and the faculty of clear vision, if prone to sin, may be the hand that causes to stumble, and the foot that causes to stumble, and the eye that causes to stumble, which things it is better to cast away, and having put them aside to enter into life without them, like as one halt, or maimed, or one-eyed, rather than along with them to lose the whole soul. And likewise in the case of the soul it is a good and blessed thing to use its power for the noblest ends; but if we are going to lose one for any cause, it is better to lose the use of it, that along with the other powers we may be saved.
And it is possible to apply these words also to our nearest kinsfolk, who are our members, as it were; being considered to be our members, because of the close relationship; whether by birth, or from any habitual friendship, so to speak; whom we must not spare if they are injuring our soul. For let us cut off from ourselves as a hand or a foot or an eye, a father or mother who wishes us to do that which is contrary to piety, and a son or daughter who, as far as in them lies, would have us revolt from the church of Christ and the love of Him. But even if the wife of our bosom, or a friend who is kindred in soul, become stumbling-blocks to us, let us not spare them, but let us cut them out from ourselves, and cast them outside of our soul, as not being truly our kindred but enemies of our salvation; for whosoever hates not his father, and mother, Luke 14:26 and the others subjoined, when it is the fitting season to hate them as enemies and assailants, that he may be able to win Christ, this man is not worthy of the Son of God. And in respect of these we may say, that from a critical position any lame one, so to speak, is saved, when he has lost a foot— say a brother— and alone obtains the inheritance of the kingdom of God; and a maimed one is saved, when his father is not saved, but they perish, while he is separated from them, that he alone may obtain the benedictions. And so also any one is saved with one eye, who has cut out the eye of his own house, his wife, if she commit fornication, lest having two eyes he may go away into the hell of fire.