[A.J.Armitage]

But when was it decided that the "Gospel of Thomas," for instance, didn't make the cut? When, where, and by whom?

By the surviving history, it was never given any serious consideration.

Because it was never used Liturgically: Lex orandi, lex credendi. Nor was it referenced as authoritative by the Ante-Nicene Fathers, or received as Scripture in Orthodox/Catholic Churches. That's the basis the Ecumenical Councils used, to determine the authenticity of Scriptures: liturgical, patristic, and ecclesial Tradition. The spurious texts were not in the Tradition.

So take the question back a step. When the Church used a book, was it recognizing something that would have been true of the book in any case, or was it giving the book something? Could the Church have used Thomas liturgically, and if it had would Thomas be Canon?

[Mrs. Don-o]

By the surviving history, it was never given any serious consideration.

Oh, it was given serious consideration by the Gnostics, all right. It wasn't accepted in the "surviving history" of the Church. "The surviving history of the Church which was handed down to us" is, in fact, the definition of Tradition.

So, In that sense, we're in agreement, are we not?

So take the question back a step. When the Church used a book, was it recognizing something that would have been true of the book in any case, or was it giving the book something? Could the Church have used Thomas liturgically, and if it had would Thomas be Canon?

Let me try to understand your question correctly: are you asking whether something, anything, could become canonical simply by being used in the Liturgy? No. If that were so, the Liturgy would be considered 100% Scripture, which it is not.

One does look at Liturgy, though, to see what the Church believes from of old. Since the Church (not individual religious enthusiasts, nitwits, and sinners, but the Church as a whole) is protected from error ("...and the gates of hell shall not prevail against it")(1 Corinthians 14:33 "For God is not the author of confusion"), what you're looking for is internal consistency, across continents, cultures, and centuries.

In the words of an ecclesiastical writer in Southern Gaul in the fifth century, St. Vincent of Lerins, here's a practical rule for distinguishing heresy from true doctrine: "quod ubique, quod semper, quod ab omnibus creditum est." What has been believed everyone, always, and by everyone.

How do you detemine that? In your words, "the surviving history." Tradition.