Are those prejudices, or convictions based on observation and judgement? Does Randi condemn out of hand, or after reasoned consideration?
I am inclined to suspect that they are prejudices. At the very least, it's hard to say on what objective observation and judgment they are based, since (as I suggested earlier) contemporary historians don't appear to share his views about the period in question. The way in which Randi speaks about the Middle Ages appears to me to reflect his having internalized the same historical myths and biases about which scholars in this area so frequently complain.
“Early Middle Ages” sounds nicer than Dark Ages – but they were dark ages, when Europe lost perhaps half its population. (At the moment I’m reading John Kelly’s The Black Death, a popular history but one that includes citations and other scholarly apparatus.)
I assume you're referring to the pandemics of the mid-6th to late-7th century, rather than the late medieval pandemic known as the Black Death. Yet I expect you'd agree that that has little to do with why Randi speaks of the Middle Ages as "dark".
Literacy was almost the sole property of the Church in those long centuries ...
It's true that following the breakup of the empire there was a decline in education, which the Church helped to counteract and eventually reverse, but for the sake of accuracy - as J.W. Thompson noted in the chapter on the early medieval period in his classic
The Literacy of the Laity in the Middle Ages - "[t]he common and widely accepted belief that in the Middle Ages no one not a cleric was able to read or to write Latin, is a gross exaggeration."
... and so was ignorance.
I'm not sure what you mean when you suggest that ignorance, like literacy, was almost the sole property of the Church.
The later Middle Ages were no better, certainly in terms of dealing with disaster. The various responses to the Black Death on the part of laity and Church, learned and simple, illustrate the helplessness of medieval society: They lit bonfires, rang bells, prayed, inhaled the scent of herbs (or latrines, a competing theory), flogged themselves bloody, and massacred Jews.
Though I agree there wasn't a great deal that could be done to combat the disease effectively, I think that your summary might paint a misleadingly (and exclusively) irrational picture. According to Prof. Darrel Amundsen's article entitled "The Medieval Catholic Tradition" in the anthology
Caring and Curing: Health and Medicine in the Western Religious Traditions (Johns Hopkins UP, 1998):
The church's reaction to the Black Death of 1348-1349, the most horrible pestilence ever to afflict Europe, is well illustrated by the actions taken by Pope Clement IV as soon as the plague hit Avignon, then the seat of the papacy. In addition to taking various measures to hinder contagion, he hired physicians to care for the afflicted, gave special indulgences to encourage the clergy to minister to those stricken, and instituted a special mass to implore an end to the plague. Medical care was viewed as an immediate need to counter this affliction ... [and] was by no means regarded as inconsistent with the equally important effort to approach God through a special mass.
One superstition was as good as another; the word “credulity” had no meaning in such a culture.
...
[Randi] does not want the world to slip back into a medieval mentality, in which nothing is known and everything is believed.
But is that really a fair characterization of a specifically medieval mentality? As the eminent historian of science Edward Grant has written (
God and Reason in the Middle Ages (Cambridge UP, 2001)):
[T]he Age of Reason began in the Middle Ages. … Modern philosophy did not have to undertake the struggle to establish the rights of reason against the Middle Ages; it was, on the contrary, the Middle Ages that established them for it, and the very manner in which the seventeenth century imagined that it was abolishing the work of preceding centuries did nothing more than continue it. …
If modern science has progressed almost unrecognizably beyond anything known or contemplated ... in the Middle Ages, modern scientists are, nonetheless, heirs to the remarkable achievements of their medieval predecessors. The idea, and the habit, of applying reason to resolve the innumerable questions about our world, and of always raising new questions, did not come to modern science from out of the void. Nor did it originate with the great scientific minds of the sixteenth and seventeenth centuries, from the likes of Copernicus, Galileo, Kepler, Descartes, and Newton. It came out of the Middle Ages ... It is a gift from the Latin Middle Ages to the modern world, a gift that makes our modern society possible[.]
Grant's point, of course - which has also been made in various ways by a great many of his colleagues - is that the reputation of the Middle Ages as an era of unreason and superstition is unmerited. A "medieval mentality" should arguably refer to "the idea, and the habit, of applying reason to resolve the innumerable questions about our world, and of always raising new questions". Instead (as used by many, including Randi), it's usually intended to convey the opposite.