Theodosius was the last ruler of the empire proper who was capable of leading his army; and from his death onwards the fall of the western section proceeded at headlong rate. His sons, Honorius and Arcadius, were worse weaklings than even the sons of Valentinian: to fit for the throne a child born in the purple, always a hard task, seemed impossible under Christianity. At the end of the fourth century begins the series of convulsions which mark the end of the Roman empire properly so-called. In the year after Theodosius' death, Alaric invaded and ravaged Greece; and, manoeuvred thence by Stilicho, proceeded to invade Italy. The tentative character of these unsuccessful first attempts, and of that of Rhadagast, only made more sure the triumph of the later; and invasion followed on invasion, till by the middle of the fifth century the West had lost Gaul, Spain, and Africa; and in the year 476 Rome, thrice sacked, received at last a barbarian king.
Through all these storms Christianity as a whole more than held its ground. The invaders were Christians, like the invaded, albeit heretics; the first conversion of Goths by the Arian Ulphilas in the previous century having been widely extended. The form of the dogma mattered nothing to the political function of the church, which was, among the barbarians as in the empire, to promote centralization up to the point at which schism became ungovernable. The Teutonic chieftains, it is clear, saw in the Christian Church a means of partially welding their peoples somewhat as Rome had been welded; and while Arianism held the ground among them, it furthered the unity that in the eastern empire was now being lost. And inasmuch as normal community of creed made possible an assimilation between the invaders and the conquered, Christianity positively facilitated the fall of the western empire. In Africa, again, where the Donatists, with their four hundred bishops, had been freshly persecuted under Honorius, the schism helped the invading Vandals, who paid for the Donatists' help by giving them freedom of worship. It is probable that the Manichæans, who were numerous in the same province, and who were also much persecuted, at first welcomed the invaders. So obvious was the risk of such alienations of heretics that Honorius, listening for a moment to the advice of tolerant pagans, went so far as to issue a law of general toleration. This, however, the orthodox clergy forced him to repeal, and the persecution of Donatists went from bad to worse. All the while the old paganism was still so common in the West that Honorius, who on the advice of his pious minister Olympius, after the fall of Stilicho, had sought to expel by edict all pagans and Arians from the service of the State, was fain later to entreat leading pagans to return. But the Arian Goths in turn showed the pagans no favour; in Greece, Alaric even broke up the Eleusinian mysteries; and the Vandals in Africa soon persecuted the Manichæans even more bloodily than they did the Athanasians, whom they went far to drive out of the province. In this way they in turn weakened their State, besides otherwise undergoing the social diseases of empire, so that in the sixth century Belisarius was able to reconquer it for Justinian, the emperor of the East. In Spain, conquered by the Arian Visigoths, there was relative toleration. The Arian clergy, however, being mostly unlettered Teutons, were less useful instruments to the ruler than Catholics could be; and late in the sixth century a new king at his accession there adopted Trinitarianism.