David's tenure coincided with the Council of Ferrara held from 1438 to 1439, at which the Georgian delegates rejected the union with the Roman Catholic Church.
The Union was ratified at the Council of Florence in 1439 which John attended with 700 followers including Patriarch Joseph II of Constantinople and George Gemistos Plethon, a Neoplatonist philosopher influential among the academics of Italy.
According to the subscription it was given in 1439 to the Library of Canons Regular at Verona by Dorotheus Archbishop of Mitylene, when he came to the Council of Florence in 1438.
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Because they did not recognize the authority of the Latin Patriarchs following the Council of Florence, the group of churchmen opposing the Council and its Union called themselves the Synaxis.
Near the beginning of the Council of Florence, on June 6, 1431 he was elected Bishop of Lausanne.