This answer is based on a number of factors which, collectively, contributed to the retreat of the teachings of Orthodox Christianity from Christ’s original covenants.

Complex Forces Behind Orthodox Christianity's Turn from Christ's Teach

This answer is based on a number of factors which, collectively, contributed to the retreat of the teachings of Orthodox Christianity from Christ’s original covenants.

Firstly, significant influence was exerted by the socio-historical and cultural-ideological differences between the eastern (Greek) and western (Latin) parts of the Roman Empire. These differences, exacerbated by political events (the division of the empire after Constantine, the final split after Theodosius, and the betrayal of Byzantium), led to the emergence of distinct directions in Christian thought and practice. As stated in one of the sources:

"Finally, among the most important factors that determined the specificity of individual teachings of ancient Christianity, one should include the relentlessly increasing divergence between Western (Latin) and Eastern (Greek) Christianity, predetermined by the initial discrepancies in the socio–historical and cultural–ideological conditions of both parts of the Roman Empire and accelerated by such political events as the first division of the empire into Eastern and Western after Constantine, its final division after Theodosius, and the betrayal of Byzantium at the moment of the conquest of the Western Empire by Germanic barbarians." (source: link txt)

Secondly, the transformation of doctrine occurred under the influence of Hellenistic philosophy, particularly Aristotelianism and Platonism, as well as theological interpretations represented by Origenism. According to Garnak, these influences essentially replaced pure Christianity in the fourth century and beyond:

"Factors by which Garnak primarily seeks to explain the origin and development of ecclesiastical-historical phenomena of those times... on one hand, he considers Greek philosophy in the form of Aristotelianism and especially Platonism, and on the other—Origenism, the theological system of Origen." (source: link txt)

Thirdly, it is worth noting the internal processes within the Church itself, where personal beliefs and cultural preferences began to influence the interpretation of the Holy Doctrine. People strived to reconcile the teaching with their own views and moral ideals, which gradually led them away from its original essence:

"For example, some Christians cannot accept the eternity of hellish torments, as their so-called intelligence does not allow them to reconcile the existence of hell with God's mercy. Others believe that Christianity can only manifest itself, say, as a monarchy. These and numerous other myths force people to conceptualize God in a way that suits their personal inclinations." (source: link txt)

Finally, the emergence of various heretical movements and sects, such as Gnosticism and Nicolaitanism, also played their role. These movements, which rejected some key elements of the incarnation and apostolic tradition, contributed to the deviation from the truth embedded in the Gospel:

"In the times of John the Theologian, just as in the 16th century, a reformation appeared in the form of a purified faith and spiritual worship of God, without any visible manifestations. This is the so-called Gnosticism, derived from the Greek ‘gnosis’ – knowledge. At the same time, there appeared offshoots of this spiritual faith, sects—docetism: those who rejected the incarnation, and Nicolaitans, who exhibited a careless neglect of the material..." (source: link txt)

Thus, the retreat from the original teachings of Christ in Orthodox Christianity is explained by a combination of external political and cultural factors, the influence of Hellenistic philosophy and orientations, as well as internal processes of adaptation and fragmentation of doctrinal teachings. All these reasons together led to the apostolic tradition being distorted and having lost its original purity.