The Donation of Constantine is a dubious document that claims to establish the authority of the Bishop of Rome - Pope Sylvester - giving him priveleges and governing powers as granted by emperor Constantine.
Protestants have often attacked the dubious authorship of this document as proof that the Papacy is a fraud.
The document itself is a forgery. But protestants want you to make the baseless logical leap of denying papal authority on the basis of this document being a forgery.
In the early 20th Century, historians revealed that the Donation of Constantine, though written at a late date - was based on facts that were documented in the authentic writings known as the acts of Pope Sylvester in the 4th Century.
So you must ask what is the precise nature of the protestant argument; is it against the document itself, or the facts contained in it? I suspect, like their predecessors centuries ago, your protestant friends are trying to smear by implication: false document = false papacy. But the privelege of the papacy in the Roman Empire was established in the 4th Century, and this is a matter of historical fact. By questioning the Donation, they hope to plant doubt in Catholic minds.
The source of my information is a very good essay by Hillaire Belloc - Survivals and New Arrivals.
