I’ve been busily reading John Philip Jenkins’ two recent histories of classical Christianity (roughly speaking from the 2nd century to the onset of Islam), The Jesus Wars and The Lost History of Christianity. Despite their florid title and marketing (and the occasional rhetorical overreach), I’ve found both books to be pretty sober critical accounts by a Christian historian who appreciates the results of those tumultuous and grievous periods. (I’ve been a bit of a fan of Jenkins, after he took his swings at the popular anti-Christian historical revisionists a few years ago. Ironically, his publisher is marketing these books, especially Jesus Wars, to the same crowd who eats up anti-Christian, or at least anti-Trinitarian, historical revisionism. 
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So far I would recommend both books as readible and detailed histories of the processing of Christian doctrine during that period. (Jesus Wars focuses more on the doctrinal side; Lost History more on the socio-cultural side. But naturally they both overlap a lot.)
There isn’t much about universalism (and its suppression or otherwise) in the books, but I did read a passing reference to something that we trinitarians ought to keep in mind: the two great “schools” of Christian thought from the 2nd century out to the schism between Eastern Orthodoxy and Roman Catholicism, were each not only defended but also founded by Christians who were also universalists (of one or another kind) in their soteriology–they believed in the full scope and persistence of God in saving all sinners, sooner or later, from sin.
One of the two great schools was the “Alexandrian”, due to being centered on Alexandria, Egypt. This school eventually came to dominate the Christian South, East and Southeast of the Empire (including Palestine). Alexandrians tended to be very mystical in their interpretations of scripture (much like Jewish rabbis), downplaying if not denying the importance of historical actions. They also tended to emphasize the unity of the human and divine natures of Christ–typically (though not always) in the direction of ignoring or minimizing the full humanity of Christ and focusing on the divinity. While Eastern Orthodoxy agreed in the eventual anathematizing of the most extreme positions of this school, the EOx religious tradition still holds more to this way of thinking, and religious devotion, than the Roman Catholics (and their Protestant offshoots) typically do; one result being that the EOx are much more focused on the “deification” of redeemed humanity than (on average) their Western counterparts. (There are mystics of the Western tradition who do go this route, of course, but not as many. The Coptic, Ethiopian, and Syrian Churches of the East, are even more strongly connected to this school.)
The Alexandrian school was more-or-less inaugurated by the Church Father Origen, who among other things also invented and/or promoted much of the technical language the Church would use for centuries afterward to discuss and debate the unities of God and of Christ, including in the extensive trinitarian debates of the late 4th century onward. Without Origen, the ‘substance’ of those debates (sorry for the pun!) might have been rather different. Origen was also a universalist.
Origen’s student a few generations later, Athanasius, would champion the full divinity of Christ (and the humanity, too, although that wasn’t his main focus) against the deacon Arius and his party, during the Nicean Council. Athanasius, so far as I know, wasn’t a universalist. But a few generations later again, the great Gregory of Nyssa, student of Origen and Athanasius, would be lauded as the “father of orthodoxy” and as “the orthodox of the orthodox” for his campaigning defense in moderation of the Two Natures doctrine of Christ from the perspective of a school that tended to (over-)stress the singular union of those natures. Gregory is still lauded as one of the last shared saints and doctors of East and West branches of Orthodoxy. And Gregory was a universalist.
The other great “school” of Christian thought during the Imperial period was the “Antiochene”, due to being centered on the upper Syrian town of Antioch. The Antiochians tended to focus more on the importance of the historical reality of the Biblical narratives, and less on esoteric, mystical, analogical and/or typological interpretations, than their brothers of the Alexandrian school. A natural result of this was that they also focused more on the human and divine identity of Christ being (in various ways) two natures. Not surprisingly, they also tended toward separating these two natures in such a way as to minimize the divine action or even reality of God (i.e. with Jesus less than fully divine).
The Antiochene school produced many champions of ‘orthodox’ trinitarianism–as can be appreciated by the fact that West and East Orthodox branches position ourselves as “Two Nature” advocates compared to the surviving remnants of “One Nature” trinitarians in Egypt, Syria and elsewhere, despite rejecting the extremities against which these Monophysites (who are, in fact, not strictly Monophysite about the nature of Christ, having also rejected the extremities of their own side). Roman Catholic and Protestant theology still, on average, hews closer to this school of Christian thought than to Alexandrian, however. The ancient Nestorian Church of the East, which still survives in remnants in Iraq and Iran, also derives its focus from this school.
The “Antiochene” school itself was inaugurated (in response to earlier overemphasis by One Nature advocates) by Diodore of Tarsus (who naturally felt an obligation to closely study St. Paul) and his successor Theodore of Mopsuestia. The teaching of both on the way the Two Natures related to one another in Christ was eventually repudiated for going too far into schism between the natures, but they set the initial standard for keeping the humanity of Christ as fully in view as His divinity; without them, it would have been far more difficult to keep from ever-more-minimizing the humanity of Christ. (As it happens, one ironic result of going too far into schisming the two natures of Christ, was also to minimize the humanity of Christ as being relatively unimportant to the action of the Father in the life and salvation of Christ. The rallying cry of orthodox moderation to either extreme can be summed up as “That which was not included, was not saved.”)
Much of the infighting among trinitarians in the classical period came from each school producing extremists having to be rebuked by moderates (and, of course, extremists) in other schools–the great theologians of ‘orthodox’ trinitarianism strove to balance the two concepts as much as possible, realizing that each side had important contributions to make to rightly preaching and representing Christ. Unfortunately, the moderation attempts of the middle-grounders still tended to be socially and culturally oppressive, even to other trinitarian Christians. Such was the spirit of the age; and many people suffered for it. 
Anyway, we haven’t mentioned Diodore or Theodore much on this forum yet (Origen and Gregory a bit more), so when Jenkins briefly mentioned them I thought I ought to drop in a mention of them for reference’s sake. 
