In order to give an air of antique legality to Pepin’s gift, churchmen forged a document, purporting to be a decree issued by the Emperor Constantine, by which, when he founded the New Rome, he bestowed upon the Pope the old Rome and all its Western territories. This bequest, which was the basis of the Pope’s temporal power, was accepted as genuine by the whole of the subsequent Middle Ages. It was first rejected as a forgery, in the time of the Renaissance, by Lorenzo Valla in 1439.

Bertrand Russell, History of Western Philosop…, loc. 1521