Christianity broke the spirit of the empire
Christianity has sometimes been blamed for weakening the previously vigorous martial spirit of the Roman Empire. It was most famously proposed by Edward Gibbon, in his Decline and Fall of the Roman Empire.
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From Constantine I onwards Rome became increasingly Christian. The Emperor Theodosius I effectively banned paganism, and the Catholic church became an increasingly powerful institution. Theodosius I, who is widely believed to have been entirely under the influence of St Ambrose, the then Bishop of Milan.  Paganism was intimately tied up with civic duties such as the building of public buildings. It was also related to the martial spirit of the Empire. By removing the framework of pagan belief, and awarding power to the Catholic church, whose concerns were more otherworldly in nature, the Roman state was undermined. The cultural shift towards Christianity created a preoccupation with death, dying, and the next life. Rome’s success as an Empire came in part from its earthy, pagan value system. The catholic church became a rival to civic institutions.
The argument that Christianity caused the fall of Rome mostly comes from Edward Gibbon's very influential history. He was writing during the enlightenment, a period of anti-religious backlash. Gibbon also dismisses the survival of the also Christian Eastern Roman Empire.  This is likely due to prejudice against what was seen as an “Eastern” civilization. The Byzantine Empire was frequently the wealthiest, most civilized and successful civilization of the Middle Ages. For much of Western history, many historians have acted as if the Byzantine Empire did not even exist. The move towards more other-worldly introspective religions was not confined to just Christianity in the Late Roman period. There were many pagan rivals to Christianity which had an impractical transcendental focus. Nobody has yet proven that an abstract set of values had any effect on the myriad economic and military causes of the fall of the West, which are better attributed to mismanagement and catastrophe. Rome fell for many reasons. Christianity did not prevent the Eastern Empire from suriving.
[P1] Paganism created the martial spirit of Rome [P2] Christianity shifted values to more other-worldly matters [C] Christianity made Rome weak and contributed to its fall
Rejecting the premises