The early heretics and schismatics confirm Catholic ecclesiology by consistently attacking it

Every major heresy of the early Church was forced to grapple specifically with the Catholic doctrines it rejected. Gnostics had to argue against the bishops because the bishops guarded the apostolic tradition. Marcionites had to argue against the Old Testament connection because the Church insisted on it. Arians had to argue against the homoousios because the Church defined it. Donatists had to argue against Roman authority because Rome exercised it.

This pattern is historically decisive. You cannot reject a doctrine that does not exist. The specific nature of each heresy’s argument tells us what the orthodox position was — because heresy is always defined in relation to an existing orthodoxy it is departing from.

The most powerful confirmation of Catholic ecclesiology comes precisely from those who left the Church or attacked it. Their arguments reveal, in photographic negative, the exact shape of the Catholic teaching they were rejecting.