The reign of the emperor Constantine (306-337) was as revolutionary for the transformation of Rome's Mediterranean empire as that of Augustus, the first emperor three centuries earlier. The abandonment of Rome signaled the increasing importance of frontier zones in northern and central Europe and the Middle East. The foundation of Constantinople as a new imperial residence and the rise of Greek as the language of administration previewed the establishment of a separate eastern Roman empire.
See my review of "Remembering Constantine at the Milvian Bridge" as the same generally applies here regarding postmodern historiography.
In addition, van Dam over-states the case against modern historians. No contemporary author I have yet read actually believes his or her narrative as a whole presents a complete and wholly factual picture. All understand they are offering a perspective. They believe it's a valuable perspective that adds to the conversation, and they use strong language to argue that point of view. However, with the exception of factual information here and there that - at least so far - can be taken for "certain," I don't know of any who represent their work as final, definitive, or absolute.
Van Dam's criticism therefore seems to be out of line, and his work - though valuable for its reminder not to depend too heavily on source material - seems redundant in light of what modern historians generally assume about their own work.
I very much enjoyed the first and last sections in this book: Section 1 - A Roman Empire Without Rome, gave me a much better understanding of how and why the focus of the empire shifted away from Rome itself to the frontier; Section 3 - Emperor and God, was fascinating. Van Dam's treatment of the relationship between Roman theories of imperial power (in a system where co-emperors, senior emperors and junior emperors were common place), and the development of the doctrine of the Trinity was completely new to me. While I'm far from persuaded by the author's premise in this regard, it was extremely thought provoking. Section 2- A Greek Roman Empire, was for me at least, a fairly unengaging slog. I would reiterate that for those interested in the history of the development of Trinitarian orthodoxy, Section 3 is a highly provocative and worthwhile read.