The mystery cults were religious orders that existed from before the time of Plato. Mystery cults held their secrets very closely, hence the name. But modern readers can piece together their secrets with the varied sources that are in existence. I have written before on what Plato has revealed about the secrets of the mystery cults, Plotinus reveals the same (700 years later):
7. Therefore we must ascend again towards the Good, the desired of every Soul. Anyone that has seen This, knows what I intend when I say that it is beautiful. Even the desire of it is to be desired as a Good. To attain it is for those that will take the upward path, who will set all their forces towards it, who will divest themselves of all that we have put on in our descent:- so, to those that approach the Holy Celebrations of the Mysteries, there are appointed purifications and the laying aside of the garments worn before, and the entry in nakedness- until, passing, on the upward way, all that is other than the God, each in the solitude of himself shall behold that solitary-dwelling Existence, the Apart, the Unmingled, the Pure, that from Which all things depend, for Which all look and live and act and know, the Source of Life and of Intellection and of Being.
The ascension of the soul to “the One” was the key tenant of most/all mystery cults. In this sense, we understand that Platonism had significant overlap with the mystery cults. By the time of Jesus, the concepts that the Greek gods were literal were mostly abandoned in favor of a being that resembled the Classical picture of God. When Greeks and Romans converted into Christianity, these pagan concepts naturally flowed into the church.