The ultimate (and perhaps most important) point of Dr. Smith's book is flatly stated on its last page: she hopes that the evidence presented in this book provides "a salutary reminder that divisions modern historians draw between the sacred and secular in the premodern world are more likely to reflect their own outlooks than those of their subjects, and that too neatly compartmentalizing the study of the past...can prevent us from appreciating the complexity of earlier worldviews" (199). Yes, just yes; a point that cannot be repeated enough. Smith stands among a veritable wave of young scholars (perhaps sometimes inadvertently) taking on and breaking down the artificial distinctions drawn in the nineteenth and early twentieth centuries. Moreover, she does this work better than most. Indeed, I can recommend that anyone-- everyone--interested in the tenth-thirteenth centuries should read this book and benefit from it. Her sources are rich and varied, and her analysis is incisive.
Smith's brief "Introduction" begins with a consideration of a capital from Notre-Dame of Beaugency depicting David and Goliath. It would seem very easy, she continues, to read this capital as preaching to the lords of Beaugency about good monks and bad knights, especially given the site of the church within the curtain walls of those lords' castle. But Smith quickly shows that these simple dichotomies are just that, too simple. Yes, the capital spoke to the violent laymen of the twelfth century but also to the clerics themselves. They defined themselves as Davids in more than one sense, fighting against both external (violent laymen) and internal (sin) enemies. In the end, the cloistered conceptualized themselves as warriors, much like their lay counterparts, but of a higher category because they rejected the latter's vices.
Chapter 1 starts at the foundation of the monastic experience: Scripture, but more importantly, the tradition of exegesis. Smith reminds us that Christian commentators had, from the earliest centuries, concerned themselves with explicating the wars of the Israelites. Heavily influenced by Origen of Alexandria, later commentators read these wars as prophetic of the spiritual battles monks waged. Chronologically surveying the exegetical tradition (though too quickly glossing over the Carolingians), Smith amply demonstrates how this tradition accreted so that by the eleventh century every biblical verse was heavy with meaning far beyond the literal. So, in a monastic world that was suffused with contact with Scripture--the liturgy, lectio, prayer, etc.--past and present came together and the monks saw the battles of the Israelites playing themselves out, once more, in the monks' own time.
Chapter 2 finds a fuzzy line dividing monks and laymen during the eleventh and twelfth centuries. Indeed, one might argue that the reformist attempt to delineate the two groups was actively resisted by both parties, at least for a time. After all, traditional family loyalties remained important even after one entered the cloister, and these loyalties only came more to the fore with the decline of monastic oblation in the period. For example, men who had left the military life to join the Cistercians could find comfort in an already-existing militant language used to describe their new life. Once more, they became tirones--warriors-in-training.
The type of warfare these new spiritual warriors were expected to fight is the subject of Chapter 3. The appropriation of military language by the first Christian communities is, of course, well-known and is an old story in the historiography, dating back at least to Adolf von Harnack in the early twentieth century, but Smith does well to tease out new, unnoticed nuance.  She shows how this language both signaled and spurred action, transforming martyrs, for example, from passive subjects to active "agents of their own spiritual destinies" (76). Monks felt they followed this same path. Even when this path was challenged, with the advent of the Crusades and a new form of "soldiery of Christ" in the military orders, the monastic sense that they were the true milites Christi was only strengthened. Laymen, no matter their intentions or actions, could never best the purely spiritual combat waged in the cloister.
The next chapter examines manifestations of this conception, specifically how monks used martial imagery in their writings. Here, again, Smith smartly reminds us of how the weight of exegesis burdens the medieval usage of biblical passages. Commentaries on Ezekiel and Revelation allegorically elided castles and monasteries and those on Psalms conflated God as war leader with the rider from Revelation. So, when we today see any of those passages, we must remember that they were meant to evoke a complex theological argument in the mind of the reader- -insistently arguing that the monks' battles were real and they ought take their salvation by force (as if by siege).
Who the monks chose as their models in conducting this war is the subject of the final chapter. Sometimes these models were saints, each and every one of whom could be seen as a warrior. Indeed, Smith points to twelfth-century instances in which authors began to invent military backgrounds for more obscure saintly patrons. Here, the prototypical soldier saint shifted from "martyr" to "convert." What's more interesting for the reader though, is her discussion of how this intellectual transition accompanied an actual shift in practice--the contemporary move from child oblation as the primary means of entering the monastery to one of adult conversion. This even opened up a space for the loricati ("mailed ones"), "virtuoso practitioners of spiritual warfare who fought the devil in physical armor" (188).  These "mailed ones" continued to operate, well into the thirteenth century, in a middle space that according to contemporary clerical reformers, didn't exist. 
Overall, I can find very little to criticize and much to commend in Katherine Allen Smith's book. That said, in a work so concerned with nuance, there seems to be an unfortunate slippage in how she defines certain Latin words. For instance, although Smith seems to conclude that a miles, agonista (62), and praeliator (82) were interchangeable terms, I wonder if that was the case. This does not take away from Smith's larger point--there could be many types of warriors, after all--but such words would have weight, accreted tradition (like scriptural passages), that would cloud easy equivalencies. Also, perhaps understandably, I do wish she spent a bit more time on the Carolingian roots of some of her ideas. For instance, she rightly points out how important Smaragdus' Commentary on the Rule of Benedict was for twelfth-century reformers (94) but this could and should be extended to other works as well. Many, if not most, exemplars of ninth- century biblical commentaries exist because we have so many tenth- and eleventh-century copies of them. This, to me, would signal a vigorous engagement with that material, and just at the onset of the period with which Smith is concerned.
Ultimately, as I suggested above, these small points do nothing to detract from the overall quality of the book. It is thorough, thoughtful, and penetrating. In so doing, it does its best work in knocking down relatively useless categories that are revealed to have been modern constructions. In other words, Smith is fruitfully questioning assumptions--something there should be more of, and not just in Medieval Studies.
1. Adolf von Harnack, Militia Christi: Die christliche Religion und der Soldatenstand in den ersten drei Jahrhunderten (Tbingen, 1905).
2. A topic more fully explored in Smith's, "Saints in Shining Armor: Martial Asceticism and Masculine Models of Sanctity, ca. 1050-1250," Speculum 77 (2008): 572-602.
3. A list of these loricati and their sources is provided in the book's Appendix 1.