Sentence:  “It is true, Messer Segretario,” said the shopkeeper, with subdued impatience. “But will you favour us by interpreting the Latin?” / “Assuredly,” said Tito. “It does but express the conclusions or doctrines which the Frate specially teaches, and which the <WSD>trial by fire</WSD> is to prove true or false.” 
Meaning of the word marked with <WSD> tags:  A test in which a person is exposed to flames in order to assess his/her truthfulness, commitment, courage, etc.