When the Roman empire became Christian, the hard fight for existence which the Church had waged with the Gnostics was not yet forgotten; still less had the Church forgotten the last bloody persecutions which the State had inflicted upon it in a kind of despair. These two circumstances would in themselves be sufficient to explain how the Church came to feel that it had a right of reprisal, and was at the same time bound to suppress heretics.
But, in addition, there had now appeared in the highest place, since the days of Diocletian and Constantine, the absolutist conception, derived from the East, of the unlimited right and the unlimited duty of the ruler in regard to his “subjects.”
The unfortunate factor in the great change was that the Roman Emperor was at once, and almost in the same moment, a Christian Emperor and an oriental despot. The more conscientious he was, the more intolerant he was bound to be; for the deity had committed to his care not only men’s bodies but their souls as well. Thus arose the aggressive and all-devouring orthodoxy of State and Church, or, rather, of the State-Church.*
Constantine, the befouler of Christianity.
A. Von Harnack, What Is Christianity? Sixteen Lectures Delivered in the University of Berlin during the Winter-Term 1899–1900 (pp. 225–226).