I'd say by survival. There were a lot of Valentinians and other believes that are now considered "Gnostic" .. big category for a bunch of smaller beliefs (many that didn't believe in the Demiurge, Monad, Sophia or other things considered Gnostic) .. a lot of them just died off.

Same with a lot of books like Shepard of Hermanos, Gospel of Mary or Thomas. Unpopular books just stop getting copied by priests. You can find many in the Nag Hammadi translations.. I still have a physical copy of that book. It's very interesting.

It's mostly the result of might makes right, trail by conquest, popularity .. really the same way States are made. You could argue that Constantine legitimized Christianity just as much as adopting Christian legitimized Constantine and the Empire. Exact same thing goes for Abd al-Malik ibn Marwan and adopting Islam for the Caliphate.