Adding more clarity to what I've already presented.
When it comes right down to it Latter-day Saints would posit that the priesthood authority of God disappeared from the earth. That resulted in the loss of revelatory guidance. This led to a drift in maintaining the core doctrines which I listed as the scriptures that can be referenced in the New Testament. To repeat what I said earlier, the introduction of Greek/Hellenistic philosophy replaced the literal meaning of the New Testament scriptures.
Concepts like the Trinity developed over centuries through councils (such as Nicea) using Greek/Hellenistic philosophical frameworks.
Priesthood and revelation had disappeared.
This is where the corruption of the true doctrine of the Godhead came into play. It wasn't necessarily a malicious cover up, but a philosophical drift away from a literal, plain reading of early Christian texts.
Regards,
MG