It is plain that St. Augustine, in his first Christian period, was deeply indebted to Plotinus, whom he knew through Victorinus and frequently quotes by name; calling him “one of those more excellent philosophers” whose doctrine of the soul is in harmony with the Prologue of the Fourth Gospel. When he came to write the “Confessions,” the glamour of the Platonic vision had begun to fade, and he was able to deal in a critical spirit with his own brief Plotinian experience of “that which Is” (VII. 17). Nevertheless, none can understand that book without some knowledge of the Enneads, from which all its finest passages are derived, and in more than one instance—especially Book VII and the celebrated tenth chapter of Book IX—closely imitated. In Augustine’s invocation of “the Beauty so old and so new,” in his description of the “Country which is no vision but a Father-land,” or of “the Light which never changes, above the soul, above the intelligence,” we see how closely he had studied them, the extent to which their language had permeated his thought. It is, however, in the tracts composed soon after his conversion—e.g. _De Quantitate Animæ_, written about A.D. 388—that their influence is most strongly marked; and the ecstatic vision of the One is definitely put forward as the summit of Christian experience. From this time onwards, the main outlines of mystical theology were more or less fixed: and since St. Augustine was one of the most widely read and deeply reverenced of the Fathers, with an authority hardly inferior to that of Scripture itself, its Neoplatonic colour was never lost. Wherever Christian mysticism passes from the emotional and empirical to the philosophic, this colour is clearly seen, and the concepts of Plotinus, more or less disguised, reappear: even in those mediæval writers who had no direct acquaintance with Greek philosophy. The immense popularity of the so-called Dionysian writings, which derive much of their doctrine through Proclus from the Enneads, helped to establish yet more firmly the Neoplatonic character of Christian and also of Sūfi mysticism. Through these writings the conceptions of the Super-essential Godhead; of successive spiritual spheres or emanations of descending splendour, intervening between the Absolute and the physical world; and of ecstatic union with the transcendent and unconditioned One as the term of religious experience, passed over from the ancient to the mediæval world. Translated from Greek into Syriac in the fifth century, they deeply affected Sūfi philosophy. They entered Western thought in the ninth century, through Erigena’s Latin translation. It is said that by A.D. 850 Dionysius was known from the Tigris to the Atlantic: and from this time onwards his influence, and through him that of Plotinus, can be traced in the spiritual literature of Christianity and Islam.