This impressive book, winner of the National Book Award, is an incisive, passionate piece of advocacy scholarship concerning the development of anti-homosexual attitudes in the pre-modern era. It's required reading mostly because of the arguments that it lays out (many of which are regrettably stretched too thin), the sources that deploys and explicates, and the fact that it was the book that really got the ball rolling on further discussion of these issues.
Boswell's main thesis is that intolerance of homosexuality began in earnest only in the 12th Century, and that homosexuality was both common and tolerated by Christianity and the Christian states prior to that time. Boswell was a convert to Roman Catholicism, and appears to have struggled mightily as a scholar to disconnect the anti-homosexual teachings of the (current) church of his day from what he perceived to have been different attitudes in the pre-medieval church -- essentially in an attempt to drive a wedge between "early" Christianity, on the one hand, and intolerance of homosexuality, on the other. The thesis suggests that such intolerance only came later, and therefore was not intrinsic or important in the earlier periods of the church (and therefore was something that could also be discarded by the church of today as something it did not view as essential in an earlier time).
Unfortunately, Boswell, in his zeal to demonstrate his ideas, regrettably either downplays most of the contra evidence, or interprets it in what can mostly be described as rather tendentious, strained and unconvincing ways. If one is looking at history more or less objectively, without a preconceived attempt to "rehabilitate" the reputation of the early church vis-a-vis homosexuality, it's very hard to accept Boswell's thesis. History records that the early Christian Fathers like Tertullian and Clement railed quite a bit against homosexuality, common as it was in the Hellenistic world of late antiquity. After the adoption by the Empire of Christianity, history again records that statutes punishing homosexuality with death or castration almost immediately began to appear in Roman legal codes, and began to be enforced. St John Chrysostom preached some of the most virulently anti-homosexual sermons in the history of Christianity already in the late 4th Century, and the Emperor Justinian instituted extremely harsh penalties against homosexuals. The historical record, viewed objectively, is reasonably clear: institutional Christianity was hostile to homosexual activity from a very early point.
Of course, this doesn't mean that Christianity, as a belief system, must be anti-homosexual. But for Boswell, that kind of thinking was not good enough because he was interested not in rehabilitating Christianity as a belief system, but in rehabilitating institutional Christianity (particularly the Catholic Church that he so loved) from its anti-homosexual history. Unfortunately, to do so, much history has to be ignored, downplayed or interpreted in very strained ways. And ultimately, this is the undoing of Boswell's thesis. When reading this book -- which glitters with erudition and scholarship -- one can't help rooting for Boswell. You *want* his thesis to be right .... but ultimately, it just doesn't convince. An objective view of history -- while remaining aloof from the question of whether anti-homosexuality is intrinsic to Christianity as a matter of faith (that's an issue for the theologians) -- clearly demonstrates that institutional Christianity has been profoundly anti-homosexual from the time it became "institutional".
Nevertheless, the book is required reading for anyone interested in these topics, mainly because it highlights the issues, frames the debate and (undoubtedly) reflects the work of a brilliant mind.