information:classification
Rise of Christianity, possible decline of the armed forces

In 313, Constantine the Great declared official toleration of Christianity. This was followed over the ensuing decades by the search for a definition of Christian orthodoxy all could agree upon. Creeds were developed, but Christianity has never agreed upon an official version of its Bible or its doctrine; instead it has had many different manuscript traditions. Christianity's disputes may have effected decline. Official and private action was taken against heterodox Christians (heretics) from the fourth century up to the modern era. Limited action against pagans, who were mostly ignored, was based on the contempt that accompanied Christianity's sense of triumph after Constantine. Christianity opposed sacrifice and magic, and Christian emperors made laws that favored Christianity. Constantine's successors generally continued this approach, and by the end of the fourth century, Christianity had become the religion of any ambitious civil official.

The wealth of the Christian Church increased dramatically in the fifth century. Immense resources, both public and private, were used for building churches, storage barns for the grain used for charity, new hospitals for the poor, and in support of those in religious life without other income. Bishops in wealthy cities were thus able to offer patronage in the long-established manner of Roman aristocrats. Ammianus described some who "enriched from the offerings of matrons, ride seated in carriages, wearing clothing chosen with care, and serve banquets so lavish that their entertainments outdo the tables of kings".

But the move to Christianity probably had no significant effects on public finances. The large temple complexes, with professional full-time priests, festivals, and large numbers of sacrifices (which became free food for the masses), had also been expensive to maintain. They had already been negatively impacted by the empire's financial struggles in the third century. The numbers of clergy, monks, and nuns increased to perhaps half the size of the actual army, and they have been considered as a drain on limited manpower.

The numbers and effectiveness of the regular soldiers may have declined during the fourth century. Payrolls were inflated, so that pay could be diverted and exemptions from duty sold. The soldiers' opportunities for personal extortion were multiplied by residence in cities, while their effectiveness was reduced by concentration on extortion instead of military exercises. However, extortion, gross corruption, and occasional ineffectiveness were not new to the Roman army. There is no consensus whether its effectiveness significantly declined before 376. Ammianus Marcellinus, himself a professional soldier, repeats longstanding observations about the superiority of contemporary Roman armies being due to training and discipline, not to individual size or strength. He also accuses Valentinian I of being the first emperor to increase the arrogance of the military, raising their rank and power to excess, severely punishing the minor crimes of the common soldiers, while sparing those of higher rank who felt able to commit shameful and monstrous crimes. Despite a possible decrease in the Empire's ability to assemble and supply large armies, Rome maintained an aggressive and potent stance against perceived threats almost to the end of the fourth century.