Sentence:  As a great public document, addressed to the whole Christian world by him who aspired to be the first ecclesiastic, we might be disposed to question its authenticity, if it were not <WSD>avouched</WSD> by the full evidence in its favour and its agreement with all the events of the period. 
Meaning of the word marked with <WSD> tags:  To confirm or verify, to affirm the validity of.