Voegelin replied in just 8 days. In his letter dated April 18, 1950, he writes: "The opportunity to speak a few deeply felt words about Karl Popper to a kindred soul is too golden to endure a long delay. This Popper has been for years, not exactly a stone against which one stumbles, but a troublesome pebble that I must continually nudge from the path, in that he is constantly pushed upon me by people who insist that his work on the “open society and its enemies” is one of the social science masterpieces of our times. This insistence persuaded me to read the work even though I would otherwise not have touched it. You are quite right that it is a vocational duty to make ourselves familiar with the ideas of such a work when they lie in our field; I would hold out against this duty the other vocational duty, not to write and publish such a work. In that Popper violated this elementary vocational duty and stole several hours of my lifetime, which I devoted in fulfilling my vocational duty, I feel completely justified in saying without reservation that this book is impudent, dilettantish crap. Every single sentence is a scandal, but it is still possible to lift out a few main annoyances."
Voegelin lists four major flaws in Popper’s work. The complete letter can be read here. He sums up his argument against Popper with these lines: "Popper’s book is a scandal without extenuating circumstances; in its intellectual attitude it is the typical product of a failed intellectual; spiritually one would have to use expressions like rascally, impertinent, loutish; in terms of technical competence, as a piece in the history of thought, it is dilettantish, and as a result is worthless."
It took Strauss a few months write a reply. He thanked Voegelin for the detailed letter on the problems in Popper’s thesis and also revealed that he had taken the liberty of showing Voegelin’s letter to an in influential colleague “who was thereby encouraged to throw his not inconsiderable influence into the balance against Popper’s probable appointment here [at the University of Chicago]. You thereby helped to prevent a scandal.”