Those of the Roman religion do not understand the nature of the biblical canon. Their official theology forces them to insist on thinking of it as "a list of books," rather than as the (divinely-inspired) books themselves. It is the books that are canonical (the rule for faith and life for all men) -- not the descriptive list that identifies which are inspired.

This may sound at first like some sort of academic hair-splitting, but this distinction figures prominently in the ongoing debate between Romanists and Protestants. From the moment Jesus told any one of his parables, they carried all the authority of God Himself. And once written down by the "finger of God," as the Scripture refers now and again to the Holy Spirit, they became canonical.

The Church condoning and sanctioning an official list of canonical books later could not make them canonical -- or they would have lacked authority for all those prior centuries in which they were preached as the Word of God (as though the Holy Spirit had no declarative authority until the Church speaking as the same Spirit led them to said so!!). This is quite backwards. It also raises the obvious question, Why do we have to wait for the Spirit to say that He said it outside the Bible when He has already said that he said it INSIDE the Bible (2 Pt 1:20, etc)? Here, Roman apologists run into the problem of an infinite regress. If the Spirit saying "He said it" inside the Bible once is not good enough, then don't we need to wait on the Spirit OUTSIDE the Bible (i.e. via the Church) to say by that institution that he had in fact so declared -- and so on?

It is the Holy Spirit who conveys authority to men -- albeit only ministerial authority like the courts have, or like that of fathers and mothers to rule in their household (not final authority, which God alone possesses) -- to the Church. And He is the One who inspired the apostles and prophets to write the books of the Bible.

It is the original Source of inspiration -- the Spirit of God -- who communicates authority to the text of Scripture -- this immediate inspiration by the Spirit of Christ -- is what gives Scripture its final (also called "magisterial") authority. And it is this authority, which makes them canonical.

So asking, "When did Luke's gospel become canonical?" represents the same question as, "When did Luke's gospel become authoritative and written down"? The biblical (and therefore correct) answer is, "Before the ink dried." The list given by the Roman Church which was by no means Roman at the time the ink of any New Testament book dried came centuries later.