Letters between John Ainsworth and Henry Ainsworth
In one of the letters, John Ainsworth, writing from Amsterdam, refers to Prester John in a rhetorical maneuver to show that Henry's supposed Catholicism was not grounded in history or reality:
If therfore you speak, let it be as the words of God; and if by his word you can say any thing to help you, show it, and by his grace, I will hear. Otherwise your assumed name Catholik, moves me no more then the name Apostolik, Pretegiani (corruptly called Prester John,) among the Ethiopians (from EEBO)