From Plotinus to Damascius [leading figures in Neoplatonic thought], Neoplatonism was always anti-Christian. Attacking the Christian Gnostics, Plotinus simultaneously combatted specifically Christian notions, as for example, that of creation....P. Hadot then notes that Neoplatonism further entered the West via Arabic literature. Arabic philosophy had become "a Neoplatonic interpretation of the works of Aristotle. . . ." He continues:
From the middle of the 4th century onward, however, Christian thought was strongly influenced by Neoplatonic philosophy and mysticism. In the East, Basil of Cesarea, Gregory of Nyssa, Synesius of Cyrene, and Nemesius of Emesa, and, in the West, Marius Victorinus, Ambrose, and Augustine, made abundant use of Plotinus or Porphyry, frequently without citing them. In the 5th century, Pseudo-Dionysius borrowed his hierarchical universe from Proclus. In the East, this direct influence of Neoplatonism continued throughout the Byzantine period, notably up to Psellus (11th century), Michael Italicos (12th century), Nicephoros Gregoras (14th century), and Gemistos Plethon (15th century). Plethon played a role in restoring Neoplatonism to the West in the course of the Italian Renaissance, at the court of the Medici. In the West, from the high period of the Middle Ages onward, Neoplatonism was accepted through the works of Ambrose, Augustine, Boethius, Calcidius, and Macrobius. In the 9th century, John Scotus Erigena translated the writings of pseudo-Dionysius and Maximus the Confessor, and, in his De divisione naturae, combined the Proclean Neoplatonism of pseudo-Dionysius with the Porphyrian Neoplatonism of Augustine.
Once it came into Spain during the 12th century, this Arabian philosophy placed Christian thought into renewed contact with Neoplatonism.Note that a dominant pagan philosophy that strongly influenced Christianity would, centuries later, seem "naturally Christian" to those steeped in Hellenized thought.
From the 12th century onward, Latin translations from Arabic or Greek gave Christian theologians a direct knowledge of Neoplatonic works. . . . Having received a strongly Platonized thought from the Christian tradition [i.e., the post-apostolic tradition - Platonized thought is not found in the Bible!], certain theologians of this era, reading these Neoplatonic texts, regarded Platonism as naturally Christian. (emphasis mine)
Neoplatonism was closely related to Platonism - a philosophical system based on the teachings of Plato. Regarding Platonism, J.O. Riedl in the article "Platonism" in the New Catholic Encyclopedia (McGraw-Hill, NY, 1967), Vol. XI, pp. 433-438, writes:
[Platonism] is also used by some for Neoplatonism, although this is more commonly considered a separate philosophical movement closely related to Platonism. Among patristic, medieval, and modern scholars, the term is generally used to designate currents of thought of Platonic origin that flourished among the Greek and Latin Fathers, among medieval schoolmen, . . . [etc.]. Not infrequently, Platonism has also influenced the elaboration of religious doctrines, and on this account is variously called Jewish, Islamic, or Christian. . . .(p. 434, emphasis mine)The story of the influence of pagan philosophy on "mainstream" Christian doctrine after the loss of prophets and apostles is long and complex, but the effort to make Christianity seem compatible with pagan philosophy - perhaps viewed as essential for the survival of the Church - rapidly accelerated the process of apostasy. The Church - or its remnants - thrived and became the political tool of emperors and conquerors, with many doctrines that truly were adapted to be compatible with pagan intellectual thought. Thus, an immaterial God without body, parts, or passions was defined, which now appeared to be more "the God of the philosophers," as Origen put it, than the God in whose physical image we were created, as early Christians and Jews believed. Creation became the philosophically appealing creation ex nihilo, which was not known among Christians in New Testament times. The perfect unity of the three distinct Beings of the Godhead became a unity of substance in the philosophical sense. Many plain and pure teachings were corrupted, though much that is good and wholesome remained. Nevertheless, there was loss of priesthood authority, of revelation, of sacred ordinances and basic teachings, and there was great need for the miracle of the Restoration that began with the Prophet Joseph Smith.
Neoplatonism, in the view of one historian, "was the last breath, the last flower, of ancient pagan philosophy; but in the thought of Augustine it became the first page of Christian philosophy" (Copleston 1:506). Apart from influences that are now recognized as Neoplatonist, however, Christian writers found much in the older Platonism that helped them in their understanding of Christian theology and much that helped them answer philosophical questions without compromising their theology [Riedl is overly optimistic here!]. They found evidence for the unity of God, preexistence of the forms of things in the mind of God, creation of the world, . . . [etc.].
The Greek apologists during the reign of Antonines were educated in the pagan schools of philosophy. They used their knowledge to point out to the emperors, themselves philosophers, that Christian doctrine was reconcilable with philosophy, and therefore not to be condemned. . . .
At Alexandria Christian scholars adapted Platonic thought to religious instruction and scriptural exegesis. (emphasis mine)