Learn English – What does “of a kind” mean

Courage is reckoned the greatest of all virtues; because, unless a man has that virtue, he has no security for preserving any other.

Samuel Johnson, quote provided by Vincent McNabb in the comments.

Courage is not simply one of the virtues, but the form of every virtue at the testing point.

C.S. Lewis, sourced on Wikiquote.

To my mind, at the testing point implies a test. There's no test if you don't feel fear. I like the M-W definition you give. Note, though, that the Cambridge dictionary says that bravery is defined by not showing fear. It says nothing about not experiencing fear.

Off the top of my head, I'll say that both a brave person and a courageous person may experience fear. The brave person won't show the fear; the courageous person might. The brave are, then, a subset of the courageous. Take that with a grain of salt: I just made it up, but it sounds roughly right to me.

it probably has the long-lost meaning of "warehouse" or a military storehouse:

1580s, "place for storing goods, especially military ammunition," from M.Fr. magasin "warehouse, depot, store," from It. magazzino, from Arabic makhazin, pl. of makhzan "storehouse" (cf. Sp. almacén "warehouse, magazine"), from khazana "to store up." The original sense is almost obsolete; meaning "periodical journal" dates from the publication of the first one, "Gentleman's Magazine," in 1731, from earlier use of the word for a printed list of military stores and information, or in a figurative sense, from the publication being a "storehouse" of information.

I think John Calvin had this usage in mind if you actually look at the full quote:

"But herein appears the shameful ingratitude of men. Though they have in their own persons a factory where innumerable operations of God are carried on, and a magazine stored with treasures of inestimable value, instead of bursting forth in his praise, as they are bound to do, they, on the contrary, are the more inflated and swelled with pride."

To clarify as requested by OP - the Institutes was written originally in Latin and translated into English by Beveridge. The full quote above seemingly lends itself to the use of "magazine" in it's old usage as we can safely eliminate the use of a gun magazine or a journal.