The question is how to become "capable and fit" for such friendship

"Angels have not, nor affect not other knowledge of one another, then they lift to reveal to one another.  It is then in this onely, that friends are Angels, that they are capable and fit for such revelations when they are offered."

     John Donne to S^r H[enry]. G[oodyer]., Letters to severall persons of honour (London, 1651), 109-110, as quoted by David Marno in Death be not proud:  the art of holy attention (Chicago:  The University of Chicago Press, 2016), 90.