The 1949 book The God That Failed contains Louis Fischer‘s definition of “Kronstadt”: a moment in which communists or fellow-travelers decide not just to leave the Communist Party but to oppose it as anti-communists.
Editor Richard Crossman said in the book’s introduction: “The Kronstadt rebels called for Soviet power free from Bolshevik dominance” (p. x).
After describing the actual Kronstadt rebellion, Fischer spent many pages applying the concept it to some to subsequent former-communists—including himself:
What counts decisively is the “Kronstadt.” Until its advent, one may waver emotionally or doubt intellectually or even reject the cause altogether in one’s mind and yet refuse to attack it. I had no “Kronstadt” for many years” (p. 204)