The Ecclesiastical History of Sozomen

Constantine the Roman emperor was angry, and bore it ill when he heard of the sufferings to which the Christians were exposed in Persia. He desired most anxiously to render them assistance, yet knew not in what way to effect this object. About this time some ambassadors from the Persian king arrived at his court, and after granting their requests and dismissing them, he thought it would be a favorable opportunity to address Sapor in behalf of the Christians in Persia, and wrote to him, confessing that it would be a very great and forever indescribable favor, if he would be humane to those who admired the teaching of the Christians under him. “There is nothing in their religion,” said he, “of a reprehensible nature; by bloodless prayers alone do they offer supplication to God, for he delighteth not in the outpouring of blood, but taketh pleasure only in a pure soul devoted to virtue and to religion; so that they who believe these things are worthy of commendation.” The emperor then assured Sapor that God would be propitious to him if he treated the Christians with lenity, and adduced the example of Valerian and of himself in proof thereof. He had himself, by faith in Christ, and by the aid of Divine inclination, come forth from the shores of the Western ocean, and reduced to obedience the whole of the Roman world, and had terminated many wars against foreigners and usurpers; and yet had never had recourse to sacrifices or divinations, but had for victory used only the symbol of the Cross at the head of his own armies, and prayer pure from blood and defilement. The reign of Valerian was prosperous so long as he refrained from persecuting the Church; but he afterwards commenced a persecution against the Christians, and was delivered by Divine vengeance into the hands of the Persians, who took him prisoner and put him to a cruel death.”

It was in this strain that Constantine wrote to Sapor, urging him to be well-disposed to this religion; for the emperor extended his watchful care over all the Christians of every region, whether Roman or foreign.