<Below I give 3 possible explanations for St. Maximos’s statements before his interrogators when asked if he would submit to Rome if she had taught Monotheletism, and then a concession>
St. Maximos most likely wrote a few documents where it is said quite explicitly that the Roman see, due to the promise of Christ, is indefectible in faith (citation below). However, it cannot be denied that in the instance of his interrogation by the Monothelite heretics under the Byzantine Emperor , where it was insisted to him that Rome had become Monothelite, he at least verbally suggested that in this scenario, however hypothetical or realistic it may have been to Maximos himself, he would refuse obedience to a heretical Pope and remain faithful to the Church Fathers and the Council of Lateran 649, which, for him, only confirmed the former.
If this second possibility is true, and it seems the most reasonable one to me, then that leaves the Roman Catholic with recognizing that St. Maximos was right to connect together the promised investiture of infallibility from Jesus to Peter with the teaching ministry of the Roman See, something far beyond what you’d find in later Byzantine texts, but that he erred in giving it a too overblown application; and also left at his disposal is the belief of the Latin West , at least from Reformation times, had taken care to distinguish between an ex cathedra teaching of a validly reigning Pope versus something less than that. In the latter mode, the Pope is open to err, and it does not ruin our doctrine. If Maximos were to have not had this distinction in mind, then one can see how a change of mind occurred. If so, does he merit the “anathema of Rome!” ? Given his circumstances, I don’t think he would be. For starters, the doctrine of Papal infallibility had not been defined yet. Secondly, he knew that Rome taught dyotheletism dogmatically @ the Council of Lateran 649, under the presidency of Pope St. Martin, who likewise would suffer martyrdom for the same doctrine. In all likelihood, he did not trust what his interrogators were telling him. Dogmas in Rome become binding and condemning laws when they are defined, not in pre-mature times. That this saintly monk of Constantinople would say the words that he did concerning the authority of Rome is not to be ignored (unless one contests the authenticity of authorship).
Thirdly, there is the possibility that Maximos was only giving credence to what he thought an impossible hypothetical for the sake of argument, thereby leaving his personal and inward adherence to Papal infallibility intact. This might seem attractive to a Catholic, but there are grounds to speculate on this. Usually when someone grants a heretical premise, in this case the dispensable nature of communion with Rome, for the sake of argument, there is subsequently a manifest turn around where the one giving the grant shows his contrary position. But Maximos sort of leaves it to his questioners that he would be willing to be excommunicated from Rome if she were to enter communion with the Monothelites. Is it possible he was merely granting the premise while inwardly believing otherwise? Sure. Now, I think there is good reason to suspect that he never disbelieved Papal infallibility, but I think its clear that he articulated a last resort when found in the spot where he needed to deal with the possibility of Rome going heretical. He may have been confirmed in his beliefs were he to found out Rome never actually did commit heresy.
Lastly, there is the possibility that St Maximos had always disbelieved Papal infallibility, his former statements either being forged by some insincere pro-Papalist or his words being typical of Byzantine flattery, and that his act of condemning a heretical Pope followed smoothly from his already held beliefs. In this case, St. Maximos is not witness at all, Catholics should stop appealing to him, and they should also begin to consider more honestly the arguments proffered contra-Papacy from the Orthodox. I am ready to accept this, but am left to wonder why the data from the 2nd possibility is to be unconsidered.