[kawaii]

While I think you can make the case that Roman Catholicism became the de facto state religion with Constantine opening the council of Nicea, (determining who could attend and paying to have them there)

Considering as paganism was alive and well (and still had an alter in the senate), Constantine had yet to be baptised, and that the only 'determining' constantine did was to invite the heads of all churches (and as many clergy and laity as they sought fit to attend) and offer to help as many as could attend do so, your case is weak.

[wmfights]

Constantine had yet to be baptised, and that the only 'determining' constantine did was to invite the heads of all churches...

He insisted that the phrase "consubstantial with the father" be inserted AND HE WASN'T EVEN A CHRISTIAN!

He arranged and paid for those he wanted to attend.

Not really a glorious moment in our history. A select group of Christians were being used for the political purpose of unifying a pagan emperor's kingdom.