How Christianity Became Rome's Religion

Start with the empire trying to end Christianity outright: churches destroyed, scriptures surrendered, clergy made to sacrifice. Its longest consequence was internal, a split over how to treat clergy who complied under pressure.

Within two years of that persecution's own end, two emperors agree a policy of open toleration. It is not, strictly, an edict, and it was not issued at Milan, but it ends the age of persecution as imperial policy all the same.

The emperor behind that policy is the first to profess the faith himself, though the ancient accounts of his own conversion disagree with each other. What he does next moves the church from tolerated to actively shaped by the state.

Constantine summons bishops from across the empire to settle a dispute the church could not settle on its own, and pays for the gathering himself. An emperor calling and enforcing a church council matters as much as anything the council decides.

The dispute Nicaea was called to settle: whether the Son was eternal and of one substance with the Father, or a creature brought into being before all ages. The council condemns this position, but the argument runs another fifty years regardless.

The formula Nicaea produces to answer Arius, built around a word, homoousios, that appears nowhere in scripture, which is exactly what makes it decisive and exactly what keeps it contested. This is the shorter, 325 original, not the creed recited today.

Two generations after persecution ends, three emperors go further than toleration: everyone in the empire is now required to hold the Nicene faith by name, and the state claims the right to define orthodoxy by reference to two named sees.

The emperor behind that law finishes what Constantine began: Nicene Christianity as the empire's required religion rather than one permitted faith among others, fixed by a further council at Constantinople in 381.