Out of curiosity, what was the sect of the priest? I've never heard a historical argument for an actual universal religion from any clergy but it's relatively common among Catholics, at least the Jesuits I've had the longest conversations with, to hold that in all world religions there is a tendency towards a Catholic (I.e universal) truth. In any event, it might be helpful to know this priests background, these kinds of arguments as put forward by a Catholic might have a different take than those of a Unitarian Universalist, or for that matter Muslims who often hold that people are born sort of "naturally" as Muslims.

Perhaps he was referring to these two writers.

An anthropologist/linguist named Wilhelm Schmidt, who died in 1954, published an idea that all peoples started with a monotheistic belief to which they then added. He wrote about this in "The Origin of the Idea of God" and "The Origin and Growth of Religion" among others.

As I recall, an historian, Albert Hyma, claimed in his books "Streams of Civilization" and "An Outline of Ancient History" that the Sumerians originally believed in one god until foreign ideas were introduced.

This is neither an endorsement nor a condemnation of their ideas, but merely a possible source for the priest's statements.