What if Constantine never converts, continuing the christian persecution.
You didn't have a continuous persecution of Christians historically, and can be roughly summarized with two imperially supported backleash on early Christianity : Decius and Diocletian's persecutions namely, which had relatively different roots but essentially basing themselves on Christians being percieved as perturbating the social order. Neither were particularily successful or even that deep, as it concerned essentially people more or less tied with imperial institutions (from the army to the administration) which wasn't that specifically targeting Christians but was efficient enough putting them out of imperial structures but not out of Roman society at the latest.
Note that Diocletian's decrees weren't that well applied when he was succeeded by Constantius in the West which, probably like his son Constantine and as Maximianus, tought the persecution was both relatively inefficient and more or less going too far both legally and politically.
Even Galerius, which not only supported a lot of Diocletian's decisions when not influencing a lot of them,
acknowledged that.
(Eventually, the most harsh parts of the Diocletian Edicts weren't really carried out, specfically the ones that focused not on Christian hierarchy but whole communities)
So while Constantine not supporting Christianity is a perfectly valid PoD, or having dying before the 310's to really root things out, the general policy of the Roman Empire in the early IVth was really tending to tolerence of Christianity as long it was clear that they were not able to present a social or institutional trouble, something that most Christians previously part of the imperial institutions were happy to provide.
This leads to a majority of christians in the empire rebelling
You did have local revolts IOTL in Turkey, Syria and Palestine which provoked harsher application of the Edicts against Christian hierarchy, and it seems that it prevented well enough (either destabilizing communities, either under a policy of religious hierarchy not to provide reasons to the emperor).
Diocletian managed at least to behead a Christian church that wasn"t really unified either dogmatically, either structurally, before the IVth century. You'd not have much ground for a general uprising on this regard.
and when they are put down they are seen as weak.
Religious perception in Roman society was significantly more sophisticated than this.
The persecution wasn't seen as particularly efficient or justified, especially as the preservation of mostly antiquated public cults practices and usages was essentially political and didn't much thrilled people (especially in parts of imperial institutions where less than archaic Romans practices, in terms of religions, was present).
When Galerius, who was a main proponent of the persecution in the late IIIrd century, acknowledge that maybe throwing decrees and persecution at Christians was not as much of a good idea than he thought, you know that you certainly had much more resilience both among Christians but Late Roman society as a whole against such a brutal intervention of the state.
The point is, the focus was less on who was weak or not structurally (pre-Constantine church couldn't even remotely be compared with the roman state anyway), but how a political decision was considered by the populations and especially its institutional and political elites : the results were less than stellar and at best viewed as wasted times and efforts that could rather be put anywhere else with more success and relevance.
Again, Christianity being considered as a licit religion doesn't mean at the latest it's going to be the main religion of the Empire (while it could remain fairly influential in eastern and central Romania).
It doesn't mean that you had to have a triumphant religion in the Empire either : it's quite possible (and likely IMO) that you'd end up with a religious mix in Romania on which Christianity (and other remarkable cults such as Manicheism or Mithra's cult) would be but a part, with maintained popular beliefs and practices being more or less tied up and removed in the same time to more philosophically and/or mystically minded religious approaches of the time, with the more politically minded return to good old times of civic sacrifices which emperors supported in turn with more new approaches (such as Solar/Apollonian cults).