Why the reports about the heretical beliefs are basically accurate

[Giuseppe]

I refer to the basic beliefs about Marcion, Valentinus and other Gnostic beliefs.

When the various Justin, Irenaeus, Epihanius etc wrote, they were doing so already from a position of relative force, when their enemies had already been confined. Force generates security and security generates a more serene objectivity on the positions of the enemy.

An example may be the representation of Hannibal by the Greek-Roman writers: the greek Polybius was surely more objective (and less propagandist) than the Roman Livy. Polybius came before Livy. But even if Livy describes Hannibal as a very bad person from a moral pov, even so he recognized his greatness in the military field.

So if the analogy is correct, even if Marcion was described as a negative person, at least the basic facts about his original belief had to be reported more accurately by the his proto-Catholic opponents. And even more so from the writers who were more distant temporally from him.