I’d say the Donation of Constantine

It was a forged Imperial Roman decree supposedly from Emperor Constantine the Great, which gave all temporal power over the Western Roman Empire to the Pope. Now, how is this important? This was how the papacy came into possession of the Lateran Palace, which was the Pope’s residence for much of history. It also served as the justification for the pope to break the formal convention of the five episcopal sees (i.e in the early church, Rome was just one of the important seats of power, the others were: Constantinople, Jerusalem, Alexandria and Antioch and all had equal authority) and declare itself supreme over the other centres. It also played an important role in the events of the Great Schism of 1054 (which formally separated the Western Latin church from the Eastern Greek church) and the investiture crisis in the 11th to 13th centuries.