While the Donation dispensed the entire Roman Empire, Church lands were not affected by it, as they had been received from Charlemagne, who himself possessed them by right of conquest.

The document was proven a fraud on two main fronts, as the discovery of the forgery predated modern paleographic science. It gets some its dates wrong - it places two people as consul who were not consul in the same year. It also uses anachronistic Latin.

The event was not particularly significant in and of itself, but it did constitute yet another erosion of papal power at a time where the papacy was being stretched fairly thin. It had only recovered from the end of the Great Schism and Conciliar movements twenty years earlier, the popes themselves were hugely decadent and were no different than secular princes in lifestyle or political involvement, and the Hussites were still festering in Bohemia. Moreover, France, Spain, and England are in effect modern nation-states at this point, lessening papal power. It was effectively a PR blow, and one which was felt especially strongly 70 years later in the Reformation, where it became a staple rhetorical attack against papal authority.

It's also significant because it's the first scientific study of a document to determine its provenance, so I have the Donation to thank for about 24 credit-hours of class work I need to do.