But who decides what counts as the written Word of God? And what those words mean?
The Old Testament was set by the Jews and Jesus and the apostles attestation. The New Testament was written from first hand account and the promise from Jesus that the Holy Spirit would “bring to their remembrance” the things that were taught and done. No one else had the promise.
>>And what those words mean?>>>p> 1 Corinthians 2:13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual. 14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned. 15 But he that is spiritual judgeth all things, yet he himself is judged of no man. 16 For who hath known the mind of the Lord, that he may instruct him? but we have the mind of Christ.
You are talking to the wrong guy - I dispute the very idea of canon as being a Roman invention. And the Spirit tells me what He will.