Ambiguity is the fortress of heretics

     John Calvin is usually credited with this, but that attribution is looking increasingly suspicious to me, having spent quite a bit of time in Past Masters' Calvin:  Works and Correspondence database ("of heretics" and "of heresy"), Google, the Hathi Trust Digital Library, the Calvini Opera Database, and elsewhere, searching on not just the English above, but also, where appropriate, on haereticorum, herétiques, castellum, propugnaculum, and forteresse (without having looked closely yet at praesidium, ambiguitas and ambiguïté, plus any further declensions, variants in the orthography of 16th century French, and/or synonyms), etc.
     Take haereticorum as it appears in the online version of the Calvini Opera Database mounted by Yale.  Searching that for haereticorum, plus (to allow for hyphenation) reticorum, ticorum, and corum, I got a total of 70 hits on the former, but saw nothing like "fortress of heretics."  Assuming that I didn't miss something, the closest I got was (I think) this, from the first chapter of the Commentary on John, at "and the Word was with God" (CO 47 =CR 75, col. 3):

And yet the ancient writers of the Church were excusable, when, finding that they could not in any other way maintain sound and pure doctrine in opposition to the perplexed and ambiguous phraseology of the heretics [(adversus flexiloquas haereticorum ambages)], they were compelled to invent some words, which after all had no other meaning than what is taught in the Scriptures.

According to the Oxford Latin dictionary, ambages is closely related, via ambigo (AMBI + AGO) and then ambiguus, to ambiguitas.
     As for the 190 relevant hits on heretiques (heretiques, retiques, and tiques, but not yet any (if any) alternative 16-century spellings (and herétiques with the acute accent returned nothing)), I saw, merely skimming the complete Context (KWIC) Report, again nothing like "fortress of heretics".
     Potential equivalents in English (a list in progress):  refuge; heresies (rather than heretics); etc.

How can great minds be produced in a country where the test of a great mind is agreeing in the opinions of small minds?

Mill's actual words:

"Is it astonishing that great minds are not produced, in a country where the test of a great mind is, agreeing in the opinions of the small minds?"

     John Stuart Mill, "Civilization" (1836), in The collected works of John Stuart Mill, vol. 18, Essays on politics and society, part 1 (Toronto:  University of Toronto Press; London: Routledge and Kegan Paul,  1977), .  Editor's note:  "Dissertations and Discussions, I (2nd ed., 1867), 160-205, where the title is footnoted, 'London and Westminster Review, April 1836.' Reprinted from L&WR, III & XXV (April, 1836), 1-28 [(p. 12, col. 2 in the American edition)], where it is headed, 'Art. I. / Civilization,' and has right running title 'Civilization' and left running title 'Signs of the Times.' Signed 'A.' Original article identified in JSM’s bibliography as 'An article entitled "Civilization—Signs of the Times" in the London and Westminster Review for April, (No 5 and 48.)' (MacMinn, 47.)"
     I should add that, after finding this in vol. 18, I did not search vols. 19-33 (though very roughly similar ideas occur in vols. 6 and 10 when searched on great minds).
     The quotation occurs as in the headline in, for example, The ordeal of Mark Twain, by Van Wyck Brooks (1920), 148.

"Let death, the last enemy, be removed, and my flesh will be for ever my friend."

"Take away death, and the body is good. Let death, the last enemy, be removed, and my flesh will be for ever my friend. Nobody, after all, ever hated his own flesh."

"mortem tolle, et bonum est corpus. detrahatur mors nouissima inimica, et erit mihi in aeternum caro mea amica. nemo enim unquam carnem suam odio habuit."

     St. Augustine, Sermon 155.15, trans. Hill, WSA III/5, 93.  I was put onto this by Alban Massie, "Augustin, les manichéens et la manducation de la chair," Communio:  revue catholique internationale 43, no. 5 (septembre-octobre 2018):  80 (75-84).  Latin from the CAG as present in Past Masters.
     "You were already consoling yourself with the reflection, 'I would indeed like my body too to be in life; but because it can't be, at least let my spirit be, at least let my soul be.' Wait, though, don't worry. . . .  In this way you will be delivered from the body of this death (Rom 7:24), not by not having a body, or by having another one, but by not dying any more" (italics mine).