When the Novgorodians turned to Poland–Lithuania for help in limiting Moscow's growing power, Ivan III and the metropolitan accused them not only of political treachery, but of attempting to abandon Eastern Orthodoxy and go over to the Catholic Church. A draft treaty between Novgorod and the Grand Duke of Lithuania and King of Poland, Casimir IV Jagiellon (r. 1440–1492), said to have been found in a cache of documents after the battle of Shelon, made it clear that the Lithuanian Grand Prince was not to interfere with the election of the archbishop of Novgorod or the Orthodox faith in the city (by building Catholic churches in the city for example.)

The Battle of Shelon was a decisive battle between the forces of the Grand Duchy of Moscow under Ivan III and the army of the Novgorod Republic, which took place on the Shelon River on 14 July 1471. Novgorod suffered a major defeat and ended with the de facto unconditional surrender of the city. Novgorod was absorbed by Muscovy in 1478.