The Roman Empire didn't codify anything. The Nicene Creed was composed by the Church in response to the Arian crisis. That the empire (through Constantine) had an interest in peace does not mean the empire performed the clarification.
First of all, the council was summoned by the emperor, organized by him, paid for by him, facilitated using public infrastructure, and personally attended by him. He might have “deferred” to the decisions of the bishops, but only happens when everyone recognizes his power to just declare things. He also banished some of the losing bishops into exile using his personal power. Whether or not it’s the “empire” or the dictatorial emperor declaring a thing is a fuzzy line, but a lot of state power was involved.
Secondly it wasn’t “the church” who declared anything, because no such singular institution actually existed to speak with one voice. Rather, the council was a self-conscious effort by Constantine to actually create a unified church, since he worried that the existing discord threatened spiritual safety of the church. Remember that Christianity had been legal for only a decade or so and it would take a while before the more underground organization could organize and finish settling old scores. Whether or not the Bishop of Rome was recognized as the leader of the church is still a hotly debated subject; he however did not actually chair the council of Nicaea. At Nicaea we’re still a century or two out from the first papal bull, 400 years or so from the first cardinals, and 700 years from the first recognizably modern papal conclave. In any case, only 300 or so of the 1,800 bishops of the empire attended.
In many ways the church became The Church by taking over the mantle of imperial authority as the Roman Empire receded from Western Europe. This is the process that gives it the power, organization, and bureaucracy to set religious policy and speak in one voice, and none of that is in place by Nicaea.
I see no attempt at nuance here, so taking this at face value, are we then to simply dismiss Pliny the Younger's writings to Trajan about persecution? Should we ignore Hadrian's note of slanderous attacks on Christians? Was Tacitus writing also 'myth' in saying that Nero blamed Christians for the fire in Rome?
...To get rid of the report, Nero fastened the guilt and inflicted the most exquisite tortures on a class hated for their abominations, called Chrestians by the populace. Christus, from whom the name had its origin, suffered the extreme penalty during the reign of Tiberius at the hands of one of our procurators, Pontius Pilatus, and a most mischievous superstition, thus checked for the moment, again broke out not only in Judæa, the first source of the evil, but even in Rome, where all things hideous and shameful from every part of the world find their centre and become popular.
https://en.wikipedia.org/wiki/Persecution_of_Christians_in_t...
This was a complex period in history, I would be wary of anyone making sweeping judgements or applying modern political coloring which acts to cloud issues rather than to illuminate history.
The time period of Secundio's tomb is around AD 60 or so. As the article at pompeiisites.org says, "During the Roman period at Pompeii, funeral rites usually involved cremation, while only small children were buried." This burial of a 60 year old man stands out for a few reasons, but one of which is that it even exists at all given that it's contrary to custom of the pre-Christian age.