Valentian was the sole non-Christian in a long line of intolerant Arian and Athanasian Christian Emperors (both Eastern and Western Roman), and also the sole seeking to re-establish freedom of religion instead of making imperial edicts to constrain this or that group. In other words, not the rule but the exception -- which is shown by his epithet in history books, "the Apostate".

Gratian was the Western Roman Emperor who, under the influence of the bishop of Milan Ambrosius, brought the final tilt of the balance of power between Athanasians and Arians with strict decrets. He found an ally in Eastern Roman Emperor Theodosius, who would eventually become a joint Emperor, and start an unparalleled persecution also under Ambrosius's influence, which made the victory of the Athanasians (i.e. early Nicean Creed Christianity) final in the Roman Empire. (Though not overall: the Germanic tribes that would take over the Western Empire were converted by Arians, and it took more than two centuries for a real final victory.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}