Lorenzo Valla was a quarrelsome scholar in the service of Alfonso of Aragon, who was then at war with the papacy, and the work he is remembered for was written with that in mind. It is nonetheless one of the founding performances of modern scholarship.
The Donation of Constantine was a document purporting to record the emperor's grant of Rome and the western provinces to Pope Sylvester and his successors. It had underwritten papal claims to temporal sovereignty for six centuries. In 1440 Valla demonstrated that it could not be genuine, and his method mattered more than his conclusion. He showed that the Latin was not fourth-century Latin — that it used words that did not exist then and constructions no Roman would have written; that it described institutions and titles anachronistically; that the events it recorded left no trace in contemporary sources; and that its account was internally implausible. The argument is philological rather than theological, and it establishes that a text carries the fingerprints of its own moment, so that anachronism is evidence. That principle is the basis of textual criticism, of the modern editing of documents, and eventually of the historical study of scripture itself.
He applied the same instruments elsewhere, with predictable consequences. His Annotations on the New Testament compared the Vulgate with the Greek and found Jerome's translation wanting in many places; Erasmus discovered the manuscript in 1504, published it, and built his own Greek New Testament on the same premise. Valla also argued that the Apostles' Creed was not written by the apostles, and questioned the authenticity of the Areopagite writings that had shaped medieval mysticism. He was investigated by the Inquisition and saved by his patron.
On Pleasure, his most provocative philosophical work, has an Epicurean speaker argue against a Stoic that pleasure is the good, and then a Christian resolve the debate by identifying beatitude as the highest pleasure — an argument that is either a defence of Christianity or its assimilation to Epicurus, and has been read both ways since.