Latter-day Saints teach that there was a need for a divine restoration of the Gospel of Jesus Christ, including the keys of priesthood authority and major offices such as apostles and prophets, due to what we call the Apostasy. This refers to a lengthy process in which divinely called leaders were rejected and lost, and in which outside and man-made doctrines and practices were introduced to the church. While mainstream Christianity preserved much of the Gospel and the scriptures, for which we are most grateful, an examination of early Christianity shows that there has indeed been a loss of many key concepts.

For example, in terms of teachings and beliefs, is a matter of record that Greek philosophy (Platonism and Neoplatonism) strongly influenced the development of “mainstream” Christian doctrine. It’s not just Mormons who say this. It is well known among non-LDS religious scholars, including those associated with mainstream Christian organizations. For example, consider the following excerpts from the article “Neoplatonism” by P. Hadot in The New Catholic Encyclopedia (McGraw-Hill, NY, 1967), Vol. X, pp. 334-336 (excerpts from page 335):

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….

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. . . .

Hadot then notes that Neoplatonism further entered the West via Arabic literature, where Arabic philosophy had become “a Neoplatonic interpretation of the works of Aristotle. . . .” He continues:

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)

Note that a dominant pagan philosophy that strongly influenced Christianity would, centuries later, seem “naturally Christian” to those steeped in Hellenized thought.

Regarding Platonism, J.O. Riedl in the article “Platonism” in The New Catholic Encyclopedia (McGraw-Hill, NY, 1967), Vol. XI, pp. 433-438, writes:

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. (p. 435, emphasis mine)

This process of making Christianity seem compatible with a dominant pagan philosophy greatly accelerated the process of apostasy, in my opinion.

There is much more to say on this topic. One useful resource isMormonism and Early Christianity by Barry Bickmore. Dig into Barry’s impressive site for a wealth of knowledge about the Apostasy.