J.B. Bury, History of the Later Roman Empire (11.3):
“Persecution was an unavoidable consequence of Constantine’s act in adopting Christianity. Two of the chief points in which this faith differed from the Roman State religion were its exclusiveness and the vital importance which it assigned to dogma. The first logically led to intolerance of pagan religions, the second to intolerance of heresies, and these consequences could not be averted when Christianity became the religion of the State. It might be suggested that Constantine would have done better if, when he decided to embrace it and favour its propagation, he had been content to deprive pagan cults of their official status and to allow Christianity to compete in a free field with its rivals, aided by the prestige which it would derive from the Emperor’s personal adhesion and favour. But such a policy would have been an anachronism. A state, at that time, was unthinkable without a State cult, and if an Emperor became a Christian a logical result was that Christianity should be adopted as the official religion of the Empire, and a second that the old Roman policy of toleration should be thrown overboard. In an age of superstition this was demanded not only in the interest of the Church but in the interest of the State itself. The purpose of the official cults in the pagan State was to secure the protection of the deities; these were liberal and tolerant lords who raised no objection to other forms of worship; and toleration was therefore a principle of the State. But the god of the new official religion was a jealous master; he had said, ‘thou shalt have none other gods before me,’ and idolatry was an offence to him; how could his protection and favour be expected in a state in which idolatry was permitted? Intolerance was a duty, and the first business of a patriotic ruler was to take measures to extirpate the errors of paganism.
But these consequences were not drawn immediately. It must never be forgotten that Constantine’s revolution was perhaps the most audacious act ever committed by an autocrat in disregard and defiance of the vast majority of his subjects. For at least four-fifths of the population of the Empire were still outside the Christian Church. The army and all the leading men in the administration were devoted to paganism. It is not, therefore, surprising that Constantine, who was a statesman as well as a convert, made no attempt to force the pace. His policy did little more than indicate and prepare the way for the gradual conversion of the Empire, and was so mild and cautious that it has been maintained by some that his aim was to establish a parity between the two religions.”