Also, I think it’s worth keeping in mind that if Christianity hadn’t become a state religion (and the state religion of more than one state eventually), we probably wouldn’t be Christians now! In fact Arian Christianity did become the state religion (for all practical purposes) during most of the 4th century (when the game starts, an Arian Christian emperor in the west has just been murdered in a coup and his Arian relative on the throne in the East is trying to figure out what to do), and many of the feudal “barbarian” states that emerged in the West after the fall of Rome were neo-Arian. But they didn’t survive as such, which is a main reason why most Christians are trinitarian now and unitarian Christians (of various varieties) are minorities. (Except for Islam in its own special-exception not-actually-Christian way.)
The Oriental Orthodox Christians (based around Alexandria) were universalistic, or have had an important history of it anyway, but they were overrun and denied state support even while in the Empire, and barely survive now. The Church of the East was massively succesful at evangelism despite a lack of official state support, and was generally universalistic; but they were progressively overrun by Mongols and Muslims and barely survive now.
How many of us are Syrian Orthodox, or Coptic Orthodox? Not many. But these were super-important and super-influential groups in their day, direct heirs of the chief catechetical schools of all Christendom–founded and run by Christian universalists! if they had had state support, Christian universalists might today be as numerous as, say, Southern Baptists.
I do think it’s interesting that in the 6th century (the 500s), having suffered 200 years of encroaching barbarian raids–raids which had long since effectively dismantled the Western Empire–it was the Emperor Justinian, a powerful ruler determined to protect the remaining Empire and to reclaim the people now under oppression by barbarian invaders, who tried to stomp out Christian universalism among the Eastern Orthodox. And largely succeeded. That the Roman Pope was willing to ratify Justinian’s anathemas is something we may regret, but at the time the Roman Catholic Church was the last unifying force of civilization in the West, and had to choose (in the shadow of Augustine’s broodings over the fall of Rome as punishment from God) how best to try to keep Christendom from lapsing either into paganism or into something they would have regarded as tantamount to paganism (the neo-Arianism of the feudal lords who regarded Christ as a mere man elevated to heroic demigod status by God. I realize the Arians on the forum won’t like that description, but that’s how the Catholics saw it. The Arians, who had been in control during most of the 300s and had now in a backhanded way triumphed at last, saw it the other way around of course: it was trinitarianism which promoted an absurd and dangerous polytheistic paganism.)
Promoting the belief that God would save those people after all despite their lack of loyalty to proper orthodox belief and “Christian” behavior, must have seemed a harder and more dangerous row to hoe; after all, hadn’t Christendom gotten into this position by being accommodating to the beliefs of pagan barbarians to begin with?! Final hopelessness provided stimulus to retake what had been lost, and to not give further ground.
This wasn’t an academic or speculative question. It was a painful emergency situation, and it wasn’t only the “hierarchs” in the church who were suffering and going to suffer if they chose a path that led to the loss quite literally of Christian civilization. (Which of course was a superior civilization having inherited and borrowed it from the Greco-Romans, but still it had brought benefits to people generally. It would do so again in the overrun East when Muslim conquerors appointed Christian and Jewish scholars as advisors and intellectuals for transmitting the benefits of culture that had been obtained in the classical world.)