Richard Carrier summarizes Plutarch's book on the mystery cult of Isis and Osiris; draws parallel between resurrection of Jesus and the euhemerization and resurrection of Osiris.
Richard Carrier, On the Historicity of Jesus: Why We Might Have Reason to Doubt (Sheffield: Sheffield Phoenix Press, 2014), 114-15
Element 14: Mystery cults spoke of their beliefs in public through myths and allegory, which symbolized a more secret doctrine that was usually rooted in a more esoteric astral or metaphysical theology. Therefore, as itself a mystery religion with secret doctrines, Christianity would have done the same.
The most explicit discussion of this fact can be found in Plutarch's book on the myths and teachings of the mystery cult of Isis and Osiris, which he wrote and dedicated to a priestess of that cult, Clea. Plutarch says the highest aim of any religion is to learn the truth behind its stories and rituals, the truth about the gods. And part of that consisted in realizing that the stories and narratives of the gods were only allegories for higher truths:
Clea, whenever you hear the mythical stories told by the Egyptians about their gods-of their wanderings, dismemberments, and many experiences like these-you must remember what I said earlier and not think that any of these things is being said to have actually happened like that or to have actually come to pass.
He then goes on to summarize what is essentially the 'gospel' of Isis and Osiris, a typical mythic narrative of events transpiring on earth leading to Osiris's death and resurrection. He then closes by repeating the point that Clea knows better than to really believe these stories, that 'in fact, you yourself detest' those who take them literally, and that she (like all true believers) sees them as ' but window dressing' that points us to something else more profound.