In the account of his trial, the Eastern authorities questioned him on what he would do if the Roman Church made any sort of agreement with the Byzantines (those who had imprisoned Maximus). Here's how it went down:
7. They said to him, "And what will you do if the Romans unite with the Byzantines? For behold, yesterday there came legates of Rome and tomorrow on Sunday they will take communion with the patriarch; it will become evident to all that it was you who turned the Romans away. Doubtless with you removed, there will then be an easy union." And he said to them, "Those who are coming cannot in any way prejudice the see of Rome, even if they should take communion because they have not brought a letter to the patriarch. And I am not at all convinced that the Romans will unite with them unless they confess that our Lord and God by nature both wills and works our salvation according to each of the natures from which he is, in which he is, as well as which he is." And they said, "And if the Romans should come to terms with them at this time, what will you do?" He replied, "The Holy Spirit, according to the Apostle, condemns even angels who sanction anything against what has been preached" [Maximus the Confessor, Selected Writings (Paulist Press, 1985), p 23].Notice at the end Maximus quotes Galatians 1:8, "But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed!" I'm not saying Maximus was a proto-Protestant, but he certainly had the right idea here about what the ultimate authority truly is.