You should read the whole quote through, as it is all one point: Gibbon's desire to "excoriate" Constantine led him not only to regard reforms generally considered today to have been anywhere from "good" or "needed" to "genius" as bad, but he also spent some time in his eloquent way and usual rhetorical flourish lambasting Constantine's decisions without reason (other than his distaste for Constantine and that period and his love for the earlier age). In what I quoted, Jordan mentions two such instances: the move to Byzantium and the military reforms, one considered a move which allowed the Empire to last (at least in the East) for a much longer period, the other a "much needed" decision. In both cases, Gibbon is at odds with modern historians, and Jordan's point is that the reason for his error here is his bias.
It looks like I might have experienced some confusion when chopping your quote up into separate sentences. It is certainly a substantive critique. I'm not saying that I agree with it, but it is something concrete (for a change).
Does Gibbon unfairly excoriate Constantine? I'm willing to admit the possibility. Was he moved by prejudice to criticize reforms that in hindsight actually deserve praise, was he carried away by his enthusiasm for eloquence and rhetorical flourishes? Perhaps. Even great scientists can suffer from prejudice.
It is a complex question. These kind of historical judgments are very problematic. It is difficult to decide on the wisdom and efficacy of certain policy decisions that are much less distant from us in time (the Vietnam War, Social Security, etc.), when we have much fresher information and many more sources to sift through. Sometimes it actually helps to let time pass, to let the party politics subside, to achieve a proper detachment. But I don't think this is one of those cases.
The party politics involved in this specific question will never subside. Everyone who critiques Gibbon's take on Constantine has to be, at least initially, suspected of a pro-Christian bias. Everyone, like me, who puts his faith in Gibbon's judgments, particularly these anti-Constantine ones, has to be suspected of some kind of anti-Christian bias. Gibbon himself falls under this cloud of suspicion.
I probably can't answer the question. I am reluctant to issue an opinion on a complex question concerning government policy (which always involves unintended consequences), until after a deep study of the problem.
I do know this, however: Gibbon has made a deep study of the problem. He was surely more informed on the subject than I can ever hope to be. You may assert that essential sources were not available in 1776, but I doubt you can claim that he was ignorant of the sources that were available.
And I am automatically skeptical of any claim that he asserted anything significant in any way, let alone a judgment on an important subject like this
"...without reason (other than his distaste for Constantine and that period and his love for the earlier age)."
I find it almost impossible to believe that Gibbon asserted anything "without reason," and more importantly, without an appeal to evidence-based reasoning. You could maybe assert that the reasoning is wrong, that the evidence is wrong, but certainly you cannot assert it is nonexistent. That's not the impression I carried away.
To further complicate the issue, these kinds of opinions and judgments on the decisions of government, even when they seem to conflict, they often simply reveal different aspects and nuances of the problem. Perhaps the move to Byzantium was indeed "the crowning political achievement of Constantine's reign, a stroke of genius that created the necessary preconditions for the survival of Rome in the East." But, that doesn't necessarily answer the question of Constantine's motives, which may still have been "the work of a pusillanimous emperor willing to sacrifice the old Rome in order to erect a monument to his own glory." I am inclined to trust Gibbon's reading of his character, but it is still possible he did the right thing for the wrong reasons, so to speak.
The military "reforms" seem very problematic to my poorly informed eye. If a result of the reform was a strengthening of "the borderers, or frontier troops" at the expense of "the palentines, or garrison troops," then I have to say, yes, on the face of it, Gibbon is right. I would expect these palentines to be better soldiers, more invested in the success of the Empire, with better discipline. The borderers sound a lot like mercenaries, and there will always be certain irreducible problems with the use of such troops.
This debate was not definitively closed by D. P. Jordan in 1969. It reminds me of the change-over in American foreign policy, which followed in the wake of Vietnam, from a draft army to an all-volunteer army. Personally, I think the wars of a republic should be fought by the rank and file citizenry, instead of professional military men. But, that is just my opinion, and Jordan's judgment on the military reforms of Constantine is also an opinion (just like Gibbon's).
Before making a more solid judgment, I would want to study the military campaigns of Rome, from both before and after the reform. I would like to know, when Jordan asserts that "[Gibbon's] attack on the military reforms... has little basis in fact," is he saying that Gibbon was ignorant of the military history of Rome? Did he misrepresent this history in any way, spin certain military successes as failures, just to make Rome's post-reform military performance look worse than it really was? Is there some important source of information on this performance which he overlooked, or deliberately passed over in silence? I seriously doubt it. In fact, I would expect the opposite: if one was seeking a deeper answer to this question, Gibbon would be an excellent place to begin, because I would imagine that he contains quotes or citations from all the relevant literary sources. I'd like you to name me one he missed.
Now, this post has already become disgustingly long, but I have to address our Newton analogy, and try to correct your somewhat mistaken idea of what constitutes an "authority" in a given field, and how these authorities are used. I am honestly grateful for these thoughtful, though erroneous, criticisms, because reflection on them helps me clarify my own position.
Okay, first of all, Newton. Newton maybe isn't much of an authority in mathematics, but he is absolutely an authority in Physics, arguably the central authority.
For example, to solve any problem in celestial mechanics, unless the velocities approach the speed of light, to even pose these problems, is to use the reasoning of Newton himself. Have the methods of calculation changed? Of course. Newton didn't use the basic Leibniz notation, let alone supercomputers. But the fundamental assumptions on which these calculations are based
are exactly the same.
There is an important reason that a professional physicist, say a guy cranking out orbital trajectories on some computer at NASA, doesn't necessarily sit around reading Newton in his off time. The method described in Newton he already knows intimately and applies every day at work. Once again, I stress that he does not use the same method of calculation; that has changed radically. But the method of reasoning that controls these calculations has changed in no significant detail.
That doesn't mean there is something wrong with this NASA guy picking up Newton in his off time. How can it hurt him? He might even gain new insight from the old expression of the method. Einstein gained tremendously from his reflections on Newton, and I'm going to go out on a limb here and guess that he was familiar with Newton in the original Latin.
There is no central authority in historical reasoning that serves a function comparable to the role played by Newton in Physics. When I assert that Gibbon remains an authority, I do not mean to assert that he is a central, controlling authority across all the sub-fields of historical research, and it is deeply unfair to say that I am. I recognize the fact that these are completely different fields of thought. It seems to me that the historian's job is so varied that it is difficult to speak of a single, unified field at all. What is the connecting link between a battle historian like S.L.A. Marshal or Keith Nolan, the statistical analysis of a Fernand Braudel, or the "story telling" method of a Henry Adams or Gibbon--except that all seek to understand the past. There is no single, controlling methodology, let alone one authority.
It seems to me that the historian has a duty to read everything relevant on the subject. So, when I am actively told to avoid Gibbon, that I shouldn't base any of my thinking upon him (even though his facts are admitted to be accurate), I start to get suspicious.
There has been concern expressed that one could, for example, form an inaccurate opinion of Constantine's reforms, from reading Gibbon (the one solid criticism, so far). I'm sure the facts will be accurate, but the opinion based on those facts might be wrong; I admit it. An excellent way to find out would begin with a careful reading of Gibbon.
Take the specific question of the military reforms: were they "good" or "bad"? If they were bad, then I presume it was because they had a negative impact on military performance. Well, that is a question that could be looked into, and I would expect that Gibbon was intimately familiar with this subject (the military history of Rome), and his comments on it will be exhaustively and accurately sourced. There's your list of source material. I seriously doubt this list has any significant additions since 1776.