Of greater interest to me at this point, however, is a remark that Brandon makes in one of his comments about the formation of the Canon:
I think it is presumptuous for anyone to say definitively why any book in the Bible was accepted into the canon, and in violation of Catholic doctrine. De Fide Catholica explicitly tells us that the Church did not pick and choose the canon; it received it and recognized it as from God. The only one who knows the full reasons why a book is in the canon is God Himself, who is the one who formed it as a gift for the Church. We do know from Scripture that all of Scripture somehow tells us of Christ, and we can get to know, in a very general and vague way, something of the divine purpose in seeing how the Holy Spirit has shaped the interaction of the Church with the book. But that's pretty much it.This is an interesting and important point. I have argued, in several posts, that the New Testament Canon in particular must be regarded as authoritative only because the Church herself has so regarded them, that is, the authority of the Church to determine and interpret the Canon of scripture is prior (both temporally and ontologically) to the authority of the New Testament scriptures themselves. This is a simple fact of the chronology involved, in my view, but of course Brandon is right that it does not follow from my position that we can say definitively why any particular text in the Canon was accepted. I say "definitively" because, of course, the obvious reason--namely, that the text is consistent with orthodox Christian belief--may not be satisfying to everybody. Brandon's point is directed at the following passage of De Fide Catholica:
These books the church holds to be sacred and canonical not because she subsequently approved them by her authority after they had been composed by unaided human skill, nor simply because they contain revelation without error, but because, being written under the inspiration of the holy Spirit, they have God as their author, and were as such committed to the church.The decree gives three reasons why any particular text is accepted into the Canon, and privileges one of them. The three reasons cited are:
 The Church subsequently approved the text.The decree does not deny that  and  are true in a historical sense, but since the Council is not infallible in empirical matters of history it wisely chose to declare only  as the unquestionable reason why the texts were received into the Canon. To say that this is the theological reason is not, of course, to deny the historical reasons. Indeed, theological reasons are often mere reverse-engineered explanations of what, in fact, happened historically. It is worth noting, however, that to say that the only unmistakable reason why a particular text is received into the Canon is because it has God as its author, is not to deny that the Church gets to, as Brandon put it, "pick and choose" the texts of the Canon. In one sense, obviously, the Church does not pick and choose the texts, but in another sense, it is equally obvious that she does.
 The text contains revelation without error.
 The text was written under the inspiration of the Holy Spirit.
The sense in which the Church does not "pick and choose" the texts of the Canon is this. God himself is speaking to his Church through the Canon of scriptures, and the Church recognizes that fact in accepting a particular work into the Canon. The Church does not determine what is infallibly true, the Church discerns it and accepts it. It is important to note, however, that the authentic scriptures, which were committed to writing by mortal men, are not the only source of divine inspiration in this mix. The Church herself enjoys a special charism whereby she is the authoritative instrument through which this discernment and acceptance takes place. The Church accepted the Gospel of Matthew into the Canon, but not the Gospel of Thomas. Any particular individual who decides for himself that the Gospel of Thomas was indeed a divinely inspired and authentic Gospel, is simply mistaken, whatever claim he might make for the validity of his private judgment in that matter.
So this, then, is the sense in which the Church does "pick and choose" the texts of the Canon: the historical process whereby a particular text is read again and again in liturgical contexts and slowly recognized as bearing the stamp of God's own truth. It is essential to recognize that this historical process would be impossible outside of the institutional structure that is the Church, because independently of her charism, anyone at any time could declare literally any text to be divinely inspired. In fact this is precisely what has happened historically, and it is the reason why there are such books as the Book of Mormon and, indeed, the Holy Qur'an.