In 1440 Italian priest, humanist, rhetorician and orator Lorenzo Valla circulated in manuscript De falso credita et ementita Constantini Donatione declamatio, proving on historical and linguistic grounds that the Donation of Constantine was a forgery. Because of church opposition the essay was not formally published in print until Ulrich von Hutten's edition of 1517. It became popular among Protestants, and an English translation was published for Thomas Cromwell in 1534. Valla argued his case so convincingly that it still stands today, and the illegitimacy of the Donation of Constantine is generally conceded.
Valla showed that the document could not possibly have been written in the historical era of Constantine I (4th Century) because its vernacular style dated conclusively to a later era (8th Century). One of Valla's reasons was that the document contained the word satrap which he believed Romans such as Constantine I would not have used.
Though initially met with great criticism, the Donation of Constantine was accepted as legitimate, especially because of the beneficial nature of its content for the western church. The Donation of Constantine suggested that Constantine I "donated" the whole of the Western Roman Empire to the Roman Catholic Church as an act of gratitude for having been miraculously cured of leprosy by Pope Sylvester I. This would have obviously discounted Pepin the Short's own Donation of Pepin, which gave the Lombards land to the north of Rome.
"Valla was motivated to reveal the Donation of Constantine as a fraud by his employer of the time, Alfonso of Aragon, who was involved in a territorial conflict with the Papal States, then under Pope Eugene IV. The Donation of Constantine had often been cited to support the temporal power of the Papacy, since at least the 11th century" (Wikipedia article on Lorenzo Valla, accessed 01-17-2009).