Why would this kook bring up priests at all, if he only wanted the Middle Ages and then “leave it at that.” Virtually every successful civilisition in history, up to and including (most liberals would argue) our own, has been deeply religious.
First, religion, and the decisions of religious leaders over the years are not one and the same. Throughout history there are examples of religous leaders meddling in politics to the detriment of local populations.
The most striking is the way Islam, through sharia law has hampered the economic, and thereby scientific development of whole populations through the centuries up to the present day.
On the Christian side of the spectrum, you have the so-called "liberation theology" whereby radical catholic priests and nuns have espoused outright Marxism in the name of social justice and taken the side of communists in places like El Salvador and Nicaragua and elsewhere in the third world.
Up until the founding of the United States, Priests were supposed to support the state in return for state forbearance. Strong man govt. (monarchy) and religous leaders (not religion itself) were often partners in crime--willingly or not. That is why we came up with the 1st amendment. Freedom of speech was not just freedom to speak out against government, but the church as well.
I would assume that is what he means. But some people have a hard time separating the religion from those who purport to speak in its name. In this analysis, priests are seen as just another meddling power broker getting in the way of individual freedom.
What he fails to point out, is that where Christianity has flourished (except in a few isolated periods and places in history), so has the freedom and liberty that makes economic and social cooperation possible.