Alright, I removed a series of answers here for spreading bad history. Let me set the record straight: There are zero records from the Council of Nicea that the council ever discussed the Christian canon. If you come here to suggest that it did, then you are going to need ample evidence.

Because some people mentioned the First Council of Nicaea, let me start with:

There is no evidence that the First Council of Nicaea discussed the Biblical canon.

The NT Canon formed pretty organically, honestly (the OT Canon was pretty well-established by the time Christianity emerged in the first place, so there wasn't much debate there). When you read the Church Fathers, they quote various books as scripture, and some of them even build lists of books they consider authoritative as scripture. One such would be Athanasius, who in his 39th Festal Epistle, written in 367, writes:

Τὰ δὲ τῆς καινῆς πάλιν οὐκ ὀκνητέον εἰπεῖν· ἔστι γὰρ ταῦτα. εὐαγγέλια τέσσαρα· κατὰ Ματθαῖον, κατὰ Μάρκον, κατὰ Λουκᾶν, κατὰ Ἰωάννην. εἶτα μετὰ ταῦτα Πράξεις ἀποστόλων, καὶ ἐπιστολαὶ καθολικαὶ καλούμεναι τῶν ἀποστόλων ἑπτά· οὕτως μὲν α. [Ἰακώβου] Πέτρου δὲ β. εἶτα Ἰωάννου γ. καὶ μετὰ ταύτας Ἰούδα α. πρὸς τούτοις Παύλου ἀποστόλου εἰσὶν ἐπιστολαὶ δεκατέσσαρες, τῇ τάξει γραφόμεναι οὕτως· πρώτη πρὸς Ρωμαίους· εἶτα πρὸς Κορινθίους δύο. καὶ μετὰ ταῦτα πρὸς Γαλάτας. καὶ ἑξῆς πρὸς Ἐφεσίους. εἶτα πρὸς Φιλιππησίους καὶ πρὸς Κολοσσαεῖς. καὶ μετὰ ταῦτας πρὸς Θεσσαλονικεῖς δύο· καὶ ἡ πρὸς Ἑβραίους· καὶ εὐθὺς πρὸς μὲν Τιμόθεον δύο· πρὸς δὲ Τίτον μία. καὶ τελευταία ἡ πρὸς Φιλήμονα. καὶ πάλιν Ἰωάννου Ἀποκάλυψις.

Again, it is not tedious to speak of the books of the New Testament. These are: the four Gospels, according to Matthew, Mark, Luke, and John. After these, The Acts of the Apostles, and the seven epistles called Catholic: of James, one; of Peter, two, of John, three; after these, one of Jude. In addition, there are fourteen epistles of Paul the apostle, written in this order: the first, to the Romans; then, two to the Corinthians; after these, to the Galatians; next, to the Ephesians, then, to the Philippians; then, to the Colossians; after these, two of the Thessalonians; and that to the Hebrews; and again, two to Timothy; one to Titus; and lastly, that to Philemon. And besides, the Revelation of John.

These are all 27 books of the current NT Canon. This is not to say that the Canon was done being argued about (heck, Luther wanted to reform the Canon as well as the Church. And you have people today who still argue about it. Particularly with the modern wrestling with the issue of gay relationships, a number of people are trying very hard right now to get the Pauline corpus ejected from the Canon). But you can see that, by the time of Athanasius' letter, it was becoming pretty well established.

This is an early example but does somewhat answer your question. During the early-medieval period monks were compiling Gospel books which were highly elaborate collections of the four Gospels, of Mark, John, Luke and Matthew, which make up much of the New-Testament. Many of these were made and soon became key to Christian ceremonies across Europe. An excellent example is the Lindisfarne Gospel which was made around the 8th century and includes all four of the gospels.

Hope this helps.

Well op, for the purpose of this question I will be coming from the position of the Council of Trent, which happened around 1546, and the declaration that those books named in said council were the canonical ones.

The first serious notion of a set of canonical books took place under the watch of St. Augustine, which evidence for the current set of canon being found in text from the council of Carthage in 397 and 419 (you can fudge the dates a little as our current calendar isn't the same, but they are close). The Vulgate, the Latin translation of the bible, pretty much sealed the books that were canonized as scripture, and those books have traditionally been accepted as canon.

As for why their are few, if any, Gnostic influences on the bible, I would direct you to look at St. Irenaeus, and the influence that he had in declaring Gnostic traditions and text as heresies.

edit: answered my own question

A similar question was asked about a month ago in this thread.

My response is here, as follows:

Speaking from the perspective of New Testament studies (probably similar for Hebrew Bible, but I can't speak to that):

There are two approaches to this question: top-down and bottom-up. I prefer the bottom-up approach, which goes basically like this:

Different communities used different sets of writings as their scriptures. As these communities came to dominate Christianity, the writings they used became the de facto scripture for the majority of Christian communities.

The only real "factor" was: if it was used, it became canon. If it wasn't, it didn't.

Later theologians and scholars paid and still pay more attention to things like edicts and councils and top-down decisions that talk about orthodoxy, inspiration, apostolicity, etc. But all of those are after-the-fact rationalizations to explain why a particular writing was already scripture for the dominant community.

For example, people could argue till they were blue in the face that the Shepherd of Hermas wasn't apostolic and therefore wasn't scripture, but that argument only held up because the majority already agreed that it wasn't scripture. If the argument were real and consistent, we wouldn't have Jude or Revelation, either. But we do. Why? Because they were used, and so they became accepted over time by the majority of Christian groups. Later rationalizing led to the idea that they were at least tangentially apostolic, and "clearly" inspired - but that's all after the fact.

And the claims for or against orthodoxy are clearly backwards. If a text is used as the basis for theological understanding by a particular community, it is by definition orthodox. So if a lot of communities are using it, then a lot of communities consider it orthodox. Writings were not included or excluded on the basis of their being or not being orthodox writings; the decision (such as it was) had already been made ages back by the fact that the communities supporting "heterodox" writings were too small to have an influence over the rest of the Christian network of churches. Heterodoxy by its very nature is defined by its being held by the losing minority.

TL;DR: There were no "rules" or "plans" in place to determine what counted as scripture and what didn't. The contents of scripture developed organically as a product of the political and social processes of the communities that used them.

A slightly more detailed (but, in my view, slightly less correct) response is the top answer in that thread.

From what I understand during my studies of Gower and Langland and my introductory understanding of manuscript production, there could be a few, or several, different factors to consider as to how the Bible, in the modern form as we see it, was compiled.

One would be to consider who the major theological thinkers of the time, the late medieval period, were referencing during their lectures. For example, often times you would see the major theologians of the 4th, 5th, and 6th centuries mentioned during these lectures because of the medieval desire to seek a harmony between Platonic and Aristotelean philosophies (these early theologians can be considered Platonic, and the scholastic period of medieval thought saw a rise in interest of Aristotelean philosophy). Some of these sources mentioned would be some such as St. Jerome or St. Augustine. These major medieval thinkers would be popular among the students and thus those who were interested in such subjects would seek out their sources. In those sources, they would find references to certain Gospels (mostly the four major ones). These students would then copy the texts. As more texts were copied, they would spread more easily. Then they became distributed more widely based on popularity. Take, for example, The Book of Hours, arguably the most popular text of the late medieval period, which drew largely on the four major Gospels.

Another factor to consider is that many texts, outside of the texts copied for student use, lecturing use, use for sermons, etc., would be commissioned by a wealthy client. These clients, again, would select these text based on popularity( or even by suggestion?) and thus propagate the distribution of texts.

And yet another factor to consider is in the very act of manuscript production. As scribes were learning their trade, they would have exemplar texts to copy from and their own pages (folios, quires, etc.) on which to practice. They could only copy what texts were available to them in whichever monastery, or what-have-you, they resided. Even these texts could be based on commission. Different monasteries had different amount of students. The larger the monastery, the more copies of certain, say Gospels, would be produced, and thus a larger distribution as the students left to pursue whatever assignment they had been given once a masters was attained.

Another factor to consider is popular authors at the time, such as Chaucer, Gower, Langland, the Pearl Poet, and others. These authors often mention the same early theologians that their contemporary philosophers also mention. This would also help spread certain major Gospels as well.

I don't know much about print production, so I cannot for certain say why some compilations have certain Gospels and others do not. But I do think that sometime in the 16th (or 15th?) century the Catholic church commissioned a printed text that is similar to our modern version of the Bible. I don't know, maybe it had something to do with the advent of humanism.

These are just a couple of ideas I have had on the same subject while studying. Most of this is based on the premise that the late medieval period saw an extraordinary desire to categorize and organize and thus lead to an increasing prevalence for textual distribution. And I guess this same ideal would have influenced the spread of the printing press. The medieval scribes loved to copy and so did the printers. Basically, social and economic factors were seem to be the most influential factors in the formation of our modern Bible.

I'm sorry I don't have any sources, I'm sure there are plenty out there. This is just my current, albeit adjacent, knowledge on the subject. edit: broke up the wall of text

Given that you are asking about Gnosticism, this is probably what you are looking for.

Here is the first prolific Christian writer to combat Gnosticism. There were earlier ones, but this guy was the most thorough. He wrote in the late 100's http://www.newadvent.org/fathers/0103.htm

The first (more or less) complete canonical list that looks the same as the NT canon today would be found in Eusebius. http://www.ccel.org/ccel/schaff/npnf201.iii.viii.iii.html in the early 300's Also worthy of note, his book, which I cited is credited with being published exactly one year before the Council of Nicea.

Also, pay attention to the way that Eusebius writes about it. He does not say "here is the official list of canonical works, sent from heaven and authorized by the Catholic church." The Catholic church was in its nascency at this time, so saying it was "officially Catholic" would not hold much sway anyway. There was not one governing body at this time who would speak with authority on this issue. Or rather, if there was the beginning of a governing body, it did not have its authority fully established yet.