Many people don't realize that a new discovery in 1945 called the Nag Hammadi library sheds light on the origins of Catholic theology, many gnostic beliefs are in the gnostic texts. You can find the the library at gnosis.org. I don't believe in the concept of later mysteries since Jesus said, "what you hear in secret declare from the rooftops" and "there is nothing secret that will not be made known". Late mysteries unfolding was not Christ's plan. All was revealed in the apostolic era.
The writings contained in this library were actually quoted in the council of Nicea concerning the origin of Christ. There are several quotes from different texts that match the councils of the catholic church. Thus, showing the councils were a mix of Gnostic thought. The Gnostics outnumbered what would be mainstream Christians at the time and Constantine invited parts of all. There are truths in these texts, but there are also teachings that are contradictions or additions to the Gospels.