Commenting on the common use of fraudulent documents to support Papal claims, one historian wrote the following about the “Donation of Constantine”:
The most circumstantial evidence for this is the famous forgery, the so-called ‘Donation of Constantine’, according to which – among other things, including a confirmation confirmation of the pope’s primacy over the four patriarchates of the east – the emperor Constantine, on transferring the capital of the empire to Constantinople in 326, handed over to pope Sylvester I dominion over ‘the city of Orme and all the places, cities and provinces of Italy and the west’. The date of this famous forgery is not known. It may have been put together under Stephen II himself, perhaps about 756, or under his successor and brother, Paul I (757-67); many historians attribute it to 774, and a recent investigation has suggested that it was added to, piece by piece, at different stages between 754 and 796. But it can scarcely be denied (though a number of historians have done so) that it reflects the situation which existed in the second half of the eighth century. Taking the evidence as a whole, it seems clear that the pope had decided that he would only be safe, and the imperial government had been expelled from Ravenna in 751, if he stepped into the exarch’s place as ruler of imperial Italy, and if, at the same time, the Lombard kingdom were reduced to its original dimensions, roughly as it was a century and a half earlier, immediately after the Lombard invasion. (Geoffrey Barraclough, The Medieval Papacy [London: Thames and Hudson, 1968, 1979], 40)
For more on the use of fraudulent documents to support the papacy, see:
William Webster, Forgeries and the Papacy The Historical Influence and Use of Forgeries in Promotion of the Doctrine of the Papacy
Catholic apologist Scott Windsor wrote a response (of sorts) to Webster, The Papacy and the Early Fathers