Of all the lost texts, the one I’d like to read most is Porphyry's "Against the Christians". It was a massive 15 volume work refuting every aspect of Christianity from both a historical and philosophical perspective. Porphyry had access to sources we don't have anymore like histories of the Phoenicians (he was from Tyre himself). Its arguments were so searing that multiple Christian emperors had it banned and every copy burned. Even mentioning his arguments was banned, so stuff Christians wrote in an attempt to “refute” it didn't survive either. We only have a few fragments via quotations, and even from what little we know, he prefigured a lot of what was later independently discovered by academic Biblical study, e.g. the Book of Daniel NOT being written during the Babylonian exile but rather, being written in the 180s BC under the reign of Seleucid king Antiochus IV Epiphanes.
I would like to read early Gnostic gospels beyond Nag Hammadi.
