"The Donation of Constantine purported to memorialize the transfer to Sylvester I and his successors of dominion over the entire Western Roman Empire for the consideration of Sylvester I's instruction of Constantine in Christianity, baptism of Constantine, and curing Constantine of leprosy. Constantine allegedly kept for himself only the Eastern Roman Empire. The forgery was probably constructed during the Frankish Papacy, when Pope Stephen II became the first pope to cross the Alps to crown Pepin the Short, who issued the Donation of Pepin (a non-forgery), granting the pope control of the lands of the Lombards, which coalesced into the first fragments of the Papal States.
It was not long before the document was denounced as a forgery, notably by Otto III, Holy Roman Emperor (r. 983-1002). By the mid 15th century, not even the popes themselves regarded the document as genuine. Italian humanist Lorenzo Valla further proved its falsity in 1440 by showing that its Latin language did not correspond to that of the 4th century. The "Donation" purports to acknowledge the primacy of Rome over Antioch, Jerusalem, Alexandria, and Constantinople, even though the last of these had not even been founded at the time of the claimed Donation.
The myth of the "Donation of Constantine" is embellished further in a 5th-century hagiographic text Vita S Silvestri (or Actus S Silvestri).Among other things, the document claims that Silvester I slew a dragon that had been threatening Rome.The text says that all of these events occurred right after Constantine's entry into the city following the Battle of the Milvian Bridge, despite the fact that Silvester I did not become bishop until 314. These events were incorporated uncritically into the Liber pontificalis c. 530. John Malalas further embellished the story in his Chronicon, which claimed that Silvester I baptized not only Constantine, but his mother Helena, and—for good measure—a large group of his relatives and Roman bystanders.Theophanes the Confessor in his Chronicle c. 815-820 adds Constantine's son Crispus to the list and viciously attacks contrary accounts as Arian lies; Theophanes refers to the Lateran Baptistery as the "Baptistry of Constantine."[
Pope Pius V's Breviarum Romanum (1568) and Pope Gregory XIII's Martyrologium Romanum (1584) also asserted that Silvester I was the baptizer of Constantine. When Pope Sixtus V erected the Egyptian obelisk to the north of the Lateran Basilica in 1588, he added to the base the inscription "Constantine was baptized here."Cardinal Cesare Baroni continued to claim this in his Annales Ecclesiastici (1592) and a French author has claimed this as recently as 1906.
This falsified version of Constantine's baptism has found its way into a great deal of ecclesiastical art.Depictions include the Stavelot Triptych (c. 1165), frescoes in I Santi Quattro Coronati, stained glass in the St Michael and All Angels' Church, Ashton-under-Lyne, and—most famously—Raphael's The Baptism of Constantine in the Raphael Rooms of the Apostolic Palace."-Wikipedia
"A pious fraud: 8th century
At the period when the popes first acquire temporal power in Italy, in the 8th century, the theory evolves that their new Papal States belong to them anyway. The first Christian emperor, Constantine, is now said to have granted to Silvester I (pope from 314 to 335) the right to rule over Italy and the whole western world.
During the 8th century, either in Rome or in the Frankish empire, this entirely false piece of history is enshrined in a document known as the Donation of Constantine - the supposed grant authorized by Constantine himself.
It is possible that this is forged in a papal context in the mid-8th century, to persuade the Frankish king Pepin III to protect Rome from the Lombards; or it may be created later by the Franks to justify their not having returned Ravenna to the the Byzantine emperor. Either way the actual writing of the document is likely to be a pious fraud, in the limited sense that the authors probably believe such a gift was made by Constantine - leaving them only with the task of providing the missing evidence.
The document is widely accepted during the Middle Ages, and is much quoted by popes to bolster their authority. It is first shown to be a forgery by a Renaissance scholar, Lorenzo Valla, in 1440. " - Historyworld.net