Boswell does indeed make that argument. Not to impugn his scholarship (being a full professor at Yale means something), but rather to point out some of his personal biases, it should be noted that he was also a strong gay rights advocate who was very much interested in both the broader social acceptance of the gay community and in legitimizing the study of non-heteronormative behaviors in medieval Europe.

With a little bit of distance from his work, it's clear that the basics of Boswell's thesis are sound: the way the Church (and western society in general) currently treats homosexuality has been anachronistically mapped onto the past as the constant state of things in Christian belief, the same assumption you admit to making. It is also clear that for the first millenium or so of Christianity, homosexuality was not a major concern in the minds of Christian writers.

However, there is a rather large gap from that to "openly tolerated." In attempting to argue this extreme, Boswell significantly overstepped the scope of what his evidence could actually demonstrate about the past, and the majority of criticisms of his book fall along these lines.

So, in short, Boswell's argument was extremely important to our understanding of the past, but, in his justifiable attempt to overthrow existing understanding, went too far.

As for why homosexuality became a big deal in the 12th century, when we find the first canonical decrees against it, this is still in part an open question. Nevertheless, we can point to some broader social trends as informative. From the first attempts to reform the papacy in the early 11th century, both clerical purity and lay religiosity started to come under direct scrutiny. These reforms, for example, were finally able to remove the institution of clerical marriage from western Christianity. The emphasis on the indoctrination of the laity also grew in this period, tied up with a closer clerical scrutiny of lay "superstition" which we can see in the writings of clerics like Guibert of Nogent, and which culminated in the decrees of Lateran IV in 1215. These decrees, among other things, mandated for the first time that all the faithful must receive yearly communion, which we take to mean that, previously, lay people did not usually commune at least once a year. On top of all of this, there was a growing obsession in the Church with heresy, although how much of this is churchmen jumping at their own shadows is also unclear - the cohesion of "heretical" groups has recently received some strong challenges in scholarship, as /u/idjet can tell you. Finally, the interest in the full and formal study of codified law only really gets going in the mid-11th century with the rise of the university.

It is hard not to see the growing need to legislate about homosexual behavior in this period as part of this interest in spirituality and purity. Therefore, it is also likely that homosexuality was similar in many respects to the other sorts of behaviors which were legislated against in this period. That is to say, behaviors which were never considered acceptable, but the prohibitions on which were only just being put into writing.

Some reading:

• Moore, Robert Ian. The Formation of a Persecuting Society: Power and Deviance in Western Europe, 950-1250. Oxford: Blackwell, 1987.

• Brown, Peter Robert Lamont. The Body and Society: Men, Women, and Sexual Renunciation in Early Christianity. Columbia Classics in Religion. New York: Columbia University Press, 2008.

I wasn’t going to touch this thread, largely because /u/telkanaru wrote such a good answer while I slept, and medieval stuff isn’t my thing. Also, I my own opinion is that a lot of Boswel’s writings are ideologically motivated, which affects his arguments to some degree. I mean, that is true of lots of historians, but in Boswell’s case I think he interprets documents about fraternal partnership to indicate acceptance of homosexual partnerships in the early church, on very scant evidence.

Anyway, I feel like I should make some comments in response to a few different posts. Mainly because some of these comments are conflating ideas and creating confusion.

/u/telkanaru says “It is also clear that for the first millennium or so of Christianity, homosexuality was not a major concern in the minds of Christian writers.”

What I take them as asserting is not about whether 1st millennial Christian writers approved or disapproved of homosexuality (I think it’s fairly clear they disapproved), but whether it was a major emphasis. It wasn’t. They didn’t make it a major part of their ethical teaching, they didn’t single it out for particular treatment. To some extent, they didn’t have to, in the sense that it was likewise not broadly condoned in society.

Compare that with the current US context – homosexuality is a major concern for Christian churches. Why? 1) Contemporary notions of sexuality define homosexuality differently to 1st millennial notions of sexuality. 2) The ethical paradigm of conservative Christian churches is, in many areas, at odds with broader society and so those differences are emphasised, 3) within the spectrum of Christian denominations this is seen as a benchmark issue to fight over.

So “major concern” and “acceptable practice” are vastly different things. I’m pretty sure that I’ve got /u//telkanaru right in that they are only discussing what was emphasised.

Secondly, the NT documents. If you scroll through /r/AskHistorians for threads on homosexuality in Ancient Greece and Rome, etc., you’ll eventually find a good discussion of how classical paradigms of sexuality are quite different from modern paradigms. “Orientation” is not a category in the 1st century. People aren’t, and don’t consider themselves “homosexual” or “heterosexual”. These are modern notions that we impose on the text. That said, the classical world is well aware of homosexual sex-acts, and one of the defining lines is active/passive roles in sex acts.

Paul uses a number of words that seem to relate to homosexual activity. In the text of 1 Corinthians 6:9 Paul refers to μαλακοί and ἀρσενοκοῖται. I would translate those terms as “passive homosexual participant” and “active homosexual participant”. Personally I don’t think there’s any doubt that the NT considers homosexual activity to be sinful. I realise that’s contested, but even with that said I don’t think it is controversial; we can argue further about this.

However, and this is pertinent, the NT documents do not treat “homosexuality” as an identity marker or anything of the sort. The treatments of homosexuality in the NT tend to fall into contexts with other kinds of sexual sin. Apart from Romans, which is doing something a little differently in terms of its rhetorical strategy, the NT is not making more out of homosexual activity as sin than it is over other forms of sexual sin.

To reiterate, I think /u/MrJ1002 is dead wrong in saying “It appears that the author(s) in the first century saw it as a type of a person, not merely an act.”

The use of nouns does not nominalise the act as a “type of person” in the way modern sexual identity theory does. No more than treating “thief” as a fundamental orientation towards property.

Well, we don't have much sources for practised homosexuality in the Early Middle Ages anyway. Best sources are Irish penitential books which include homosexuality. In these books, male homosexual acts are seen as sinful, but not more than other sexual sins. Indeed, in the high Middle Ages the punishment for homosexuality was aggravated.