WI Constantine never endorses Christianity

What if Roman Emperor Constantine decides to endorse some kind of sun worship as the official state religion instead of Christianity? A couple of other emperors had done this, and it hadn't really caught on, so let's say that it doesn't for Constantine either. Christianity remains an increasingly popular religion that is divided into a number of different groups. There are occasionally clashes with Imperial authorities, but nothing like Diocletian's earlier persecutions. Assume also that Constantine's immediate successors, whether they're his sons or otherwise, also don't support Christianity.

What happens now? Christianity is NOT an official faith, patronized by the enormous resources of the imperial state. On the other hand, you don't have the state throwing its power behind enforcing one particular version of Christian theology and turning its power against all other forms as "heretics". You don't have government officials jumping on the Christian bandwagon and at the same time infusing the church a propensity towards more hierarchy and more bureaucracy and less power for "ordinary people" , less autonomy for each congregation, and less open debate between believers about controversial issues within the church. Presumably different groups of Christians will continue to vocally criticize each other for corrupting the faith, and Christians will still clash with non Christians, but there won't be government support for one group over all others. Would this state of affairs eventually end when another emperor becomes Christian anyways? Was Christianity likely to become the predominant religion of the Roman world eventually even without official support? Is there a chance that the Mediterranean could remain a very multi-religious society, with many different groups of Christians and many non-Christian religions competing for converts and prestige?
 
Christianity as envisaged by the Apostles ?

In TTL without official imperial support for what became the state religion of Christianity, could the Christian church then have developed as the decentralised, grassroots faith envisaged by the Apostles and discussed in the New Testament, as opposed to the institutionalised, dogmatic centralised religion which the church developed into under the emperors ?
 
My Roman catholic Church would exist, and it could be better than OTL. A lot of its problems came about from being the political power of Western Civilization for a dozen centuries or so...
 
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