"any utterance TODAY that attempts to institute a new rule of faith or practice is false prophecy on its face. For that would violate Sola Scriptura."
TheoJunkie was asking me to confirm my agreement (or was confirming his agreement with me - I don't want to be stickler about the purpose of the comment), but I'm afraid I still don't understand his position well enough to know whether I agree.
One aspect of Sola Scriptura is that Scripture alone is an infallible rule in matters of doctrine and the only conscience-binding authority in matters of practice. In other words, any doctrine that is not rooted in Scripture is not properly dogma, and any rule of behavior that is not rooted in Scripture is a matter of preference.
But this doctrine is not in a vacuum. There is a reason for this doctrine. The reason for this doctrine is the perfection of Scriptures. Scriptures have always been the supreme authority even in the age of the prophets, but the word of God from the mouth of a true prophet was as infallible as the same word from the pen of the same prophet. Furthermore, Peter (for example) received new rules of behavior in a vision from God and properly conveyed them to the church before Luke completed the Acts of the Apostles in which they were recorded. Same goes for the counsel of Jerusalem.
Sola Scriptura is a practical reality more than a positive doctrine. Scripture is supreme because Scripture is known (by faith) to be the Word of God. Thus, both Paul's writings and preaching were subject to confirmation by the Word of God.
Indeed, precisely because of the primacy of Scripture, we can reject Mohamed as a false prophet because of his denial of the death and resurrection of Christ, without regard to a view on cessationism and even without regard to the fact that Mohamed lived after the time that Scripture was completed.
Similarly, we can reject the Gnostic's writings (even those purporting to be early) not simply because of the historical evidence that they were written much later, but also because they are inconsistent with the known (by faith) Scriptures.
Ever since Moses was given the law (part orally and part in writing) and placed the unwritten portion in writing, that writing has been the measure of all prophecy. The Scriptures grew as more revelation was provided. The Scriptures are now complete.
But if God speaks today, what (except for an a priori commitment to Sola Scriptura) would constrain him to reveal nothing new in terms of doctrine or practice? Presumably nothing.
But then why commit to Sola Scriptura a priori? What is the justification for Sola Scriptura if there are still prophets of God?