Lorenzo (or Laurentius) Valla (c.1407 – August 1, 1457) was an Italian humanist, rhetorician, and educator. He is best known for his textual analysis that proved that the Donation of Constantine was a forgery.
From the Wikipedia page 
Valla's originality, critical acumen, and knowledge of classical Latin style were put to good use in an essay he wrote between 1439 and 1440, De falso credita et ementita Constantini Donatione declamatio. In this he demonstrated that the document known as the Constitutum Constantini (or "donatio Constantini" as he refers to it in his writings), or the Donation of Constantine, could not possibly have been written in the historical era of Constantine I (4th Century), as 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.( refers) The document, though met with great criticism at its introduction, was accepted as legitimate, in part owing to the beneficial nature of the document for the western church. The Donation of Constantine suggests 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 consigned to the Church the Lombard holdings north of Rome.
Valla was motivated to reveal the Donation of Constantine as a fraud by his patron 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.
The essay began circulating in 1440, but was heavily rejected by the Church. It was not formally published until 1517. It became popular among Protestants. An English translation was published for Thomas Cromwell in 1534. Valla's case was so convincingly argued that it still stands today, and the illegitimacy of the Donation of Constantine is generally conceded.