Donation of Constantine

The donation of Constantine is a forged document supposedly written by the 4th-century emperor Constantine on his deathbed that transferred the whole western part of the Roman Empire, including Rome, to the Pope. This decree has been used for centuries by the Church to justify its authority over most of Europe and it wasn't proved wrong to be a forgery until around 1450 by the Italian humanist and priest Lorenzo Valla. The legitimacy of this document caused a lot of problems in relations both within the Church (ultimately causing the schism between west and east) and with other secular powers.