One of the stock objections to sola Scriptura is that Biblical teaching is subject to interpretation. Therefore, over and above Scripture itself, we requires an authoritative interpretation of Scripture actually teaches. Hence, the necessity of infallible teaching organs like ecumenical councils. Ironically, Catholic epologist Jonathan Prejean sabotages that argument by citing a truly embarrassing counterexample:
"It seems to me that if you really wanted to go after the infallibility of Nicaea, then you could point out that many of the 318 (or so) bishops who voted FOR the Council later ended up espousing an Arian interpretation, which is exactly what they thought they were supporting when they agreed to the words (or at least, they supported them only with mental reservations to placate the Emperor). Rather than simply pointing out the change in creeds, you could point out that bishops freely present at the ecumenical council had their own position later treated as an authority to contradict that same position!"