I'm not sure why you're maintaining that "Julians religion" was a separate thing when we know he converted to Iamblichan Platonism. See
Julians gods page 3:
The decision had important consequences. Julian quickly became familiar with a circle of pagan Neoplatonists headed by a former student of the 'godlike' Iamblichus, and underwent a theurgic initiation at the hands of one of them, Maximus of Ephesus. Though he kept the matter secret for a decade, he came to regard 351 as the year of his' conversion' to paganism and his awakening to Iamblichan Neoplatonism and theurgy: these commitments were never to waver, and from then on he revered Maximus as an intimate friend and mentor.
I haven't encountered this idea that Julian created a new religion in any scholarly work I've read. What's your source for that?
Establishing a hierarchy of priests doesn't mean that he created a separate religion. Julian appointed many of his Iamblichan Platonist friends as high priests. It's simply a sensible administrative arrangement in keeping with the general administrative centralization started by Diocletian. See
The ‘Pagan Churches’ of Maximinus Daia and Julian the Apostate by Oliver Nicholson:
Historians have often stressed the similarities between the priests of Maximinus and those of Julian and the Christian clergy. Maximinus 'planned to create a pagan Church',18 and his reform 'must have served as a precedent for that of Julian'.19 These comparisons have centred on the organisation of the hierarchies. Maximinus 'imite, comme le fera plus tard Julien, l'organisation de l'eglise chretienne';20 Julian's 'ordered hierarchy... is clearly in imitation of that of the Christian Church'.21 Both emperors 'had noticed how the compact, hierarchic structure of the Church gave it combative superiority'.22 However, we have already suggested that in this respect Julian's hierarchy owed less to the Christian model than it did to the well-established network of provincial priesthoods inherited from Antiquity. It is likely, a fortiori, that Maximinus also built on existing institutions more than he copied those of the Christians; his priests were 'no doubt the presidents of the provincial diets'.23
Now, putting aside Maximinus Dias for a moment, Julians high priests
were encouraged to imitate the Christians in the cultivation of personal piety and charity. I don't see why you can't be a Platonist and also be pious/charitable. Same for writing works explaining Platonism for the uninitiated (I assume by "catechism, you mean
Concerning the Gods and the Universe). I'm not sure what you mean by a set of "centrally-defined doctrines".
We don't actually know how the general pagan population reacted to Julian's religious policies. We do know that Antioch was pissed and the pagans there didn't seem to care all that much but as
Julians Gods says, we should be very wary of generalizing one city to the entire empire. There's also some criticism of Julian by some historians for being too eager with sacrificing but again, we should be careful with potentially baseless generalizations. We also know that there were multiple instances of pagan populations rioting on their own and murdering Christian bishops, with Julian chastising them but not really stopping it. Clearly at least some pagans didn't like the Christians. And of course, there are Julians Platonist friends who clearly supported him and payed for it when he died. We must also pay attention to the pagan inscriptions made in the Empire congratulating Julian:
But Bidez gave less weight than was due to the striking titles of Julian in some of the pagan inscriptions he adduced. 'To the Restorer of the Cults'; 'To the Lord Julian, Born for the Good of the State ... on Account of the Wiping Away of the Ills of the Former Time'; 'Restorer of Liberty and the Roman Religion'.133 A hundred years before, the persecutor Decius had been hailed as 'Restorer of the Cults'; now, Julian was 'Restorer of the Temples'.134 Titles of this sort respond to more than a mere declaration of religious toleration ensuring that paganism should no longer be hamstrung in competing for men's sympathies. They anticipate, or recognize, a concrete plan to weigh the balance decisively in its favour.
And here's another good quote:
'A devout and sincere attachment for the gods of Athens and Rome constituted the ruling passion of Julian.'!9 In my view, that judgement deserves to stand. As an Iamblichan Neoplatonist, Julian could subscribe to the doctrine of a transcendental First Principle as the sole true source of the real, 'known to the blessed theurgists' alone; but alongside his philosophic monism we find in his writings the traces of an irreducibly polytheist sensibility, with firm roots in ancestral patterns of pagan belief that were far from moribund.20 The key fact of Julian's devotional life is his assumption that a multiplicity of gods was constantly being manifested in the world of men, and that they must be honoured and rendered propitious by acts of cult performed in accordance with established custom. In his case, it is true, we must allow for a special factor! a pagan who had converted away from a Christian upbringing was no doubt likely to stay inclined to systematized expressions of belief. None the less, the assumptions Julian made about the gods and the forms of worship due to them were not the assumptions of one irreversibly permeated, despite his best endeavours, by a Christian education: rather, they mark the gulf which came to separate the mature Julian from the faith into which he had been born. Nor did they make for an innovatory attempt to transform the paganism of his subjects into a 'monotheistic universal faith'. Julian's promotion of paganism was first and foremost what the inscriptions declared: a restoration of the temples and cults of the ancestral gods whose worship his predecessors had sought to check by edict and law.2! And this in turn implies that the ascription to him of a totalitarian religious ideology is subject to a major proviso. His anti-Christian programme was indeed an attempt to consign Christianity to cultural oblivion: but he was not out to impose a uniform pattern on pagan thought and practice; 'He did not feast some [gods] and ignore others,' recalled Libanius, 'but made libation to all the gods whom the poets have passed down, ancestral parents and their offspring, gods and goddesses, ruling and ruled ... worshipping the different gods at different times'.22
Julian's intolerance of Christianity stemmed from a sense of outrage at those who denied the existence of the many gods and did their best to obliterate the worship of them. His determination to strip the Christian movement of the power and influence it had gained in the wake of Constantine's conversion led him to discriminate actively, in some fields anyway, against those who professed the faith. The education edict evoked a protest even from an Ammianus, and we may suppose that many other pagans may have shared his reservations. Conceivably, Ammianus' complaint was emblematic of a deeper disquiet at the degree to which religion had impinged on Julian's public policy, a sense that the rift between pagan and Christian had been exacerbated unnecessarily.23 But if the complaint went deep, it bore upon an aspect of Julian's religious programme, not the whole of it. It need not imply any lack of sympathy with his basic wish to restore the cults to their places of honour, and it gives no cause to suppose that the attempted restoration was a freakish episode which 'perplexed rather than inspired the majority of surviving pagans'.24
Whether or not the attempt had any real chance of making a lasting political impact is quite another matter, and it is not the subject of this book. In the logic of counterfactuals and the state of the evidence, the question can yield no certain answer. We are presented with the brute facts that the early fourth century saw the coming to power of a Roman Emperor determined to promote the Christian and to harm the pagan cause, and that a century later the number of those who professed Christianity had grown remarkably - from five to thirty million, on a recent guess.25 On one celebrated view, 'Constantine's revolution was perhaps the most audacious act ever committed by an autocrat in disregard and defiance of the vast majority of his subjects.'26 There is a sense in which that statement is deeply misleading, but it can still serve to warn us against any easy assumption that 'paganism' was already doomed by the time Julian reigned. If we cannot quantify the relative importance of the factors that led to its demise, there is no ignoring the fact that thirty years after Julian's death, one of them consisted in the coercion of pagans by force, the smashing of temples and their altars and statues, and on occasion the torture and killing of those who held fast to them.27 It may be inferred, at least, from the content and tone of numerous inscriptions set up in Julian's honour - and from the outbursts of pagan violence in several cities in his reign - that his pagan activism struck a deep chord in the minds of some of his subjects. The times were not gentle.
It's simply not true that the religion "withered away" once he died. It became less prominent sure but that's to be expected when every single emperor afterwards was a committed Christian who engaged in active discrimination. As I mentioned in my previous post, the Platonists continued to survive, study, and intellectually develop their doctrines and they clearly considered Julian to be one of their own. Hell, they literally dated their years by his reign! It took Justinian closing the schools and suppressing all sources of funding for the religion to finally die out, and even then, the city of Harran managed to preserve its Platonist paganism until the Islamic period.