The Church did not immediately realize that inspired Scripture was being composed within its own ranks. Originally the Christian Church saw itself as a part of Judaism. As such it recognized the authority of the Hebrew Scriptures (a.k.a., Tanakh). It also saw revelation from God as not being limited to what was written in Scripture because the religion of the Jews came before any texts were composed by them. Scriptures were but a product of their religion, not the basis for it. Some of God’s revelation was in written form, yes, but it also carried through in the very culture of the commonwealth of Israel in several ways:
Theophany: This is a physical manifestation of the Divine, such as when Abraham was visited by God in various forms, when Moses saw the Burning Bush, or when the nation of Israel stood at the foot of Mount Sinai and saw the conflagration at its top.
Oracle: When a prophet’s vision, dream, or message came to a servant of God for public proclamation to God’s people, whether or not this message was written down after it was pronounced or limited to either written or oral form.
Tradition: Jews held that God explained the Law he gave to Moses in details that were not put to writing. Add to this the way Providence shaped the culture of the Jews and their worship (such as to collect certain writings that would later become the Hebrew Scriptures or Tanakh) or the shaping of history in the Jews’ favor (such as the events that led up to what is now celebrated as Hanukkah) came to be seen as a means of revelation.
Christianity originally recognized all three, but added another:
Epiphany: Having a revelation of God in a person (Christ) or persons (the poor, sick, thirsty, etc.).
Christianity would also develop its own Tradition based on the teachings of the Apostles.
There was never an idea that Scriptures were a sole or superior form of revelation from God, not among the Jews or early Christians, until the Gnostics came into the picture. In very basic terms, the Gnostics claimed that salvation from God was limited to those who were selected by Heaven to be enlightened by God. Gnostics believed this enlightenment came to them *through written documents *associated with magic or religions. This of course was considered heresy by the Church.
But along came Marcion of Sinope (circa 85-160 C.E.), once an important figure in the Church, who liked the Gnostic idea and converted it to something that actually had some appeal to more than a few Christians. Marcion taught that Gnostic enlightenment was limited to a select few, but that all could be privy to this knowledge by following this group of chosen ones (like himself). The elect were still enlightened by written texts (the ultimate form of revelation to Gnostics), but the written texts in this case would be Christian.
Marcion developed a “rule” (in Greek, “canon”) by which to determine which Christian texts could enlighten others and which could not. His “rule” rejected the Hebrew Scriptures altogether and allowed for only a specially edited form of the Gospel of Luke and some of the epistles of Paul. Alongside this Marcion developed what may be the earliest “proof text” arguments from Scriptures to “prove” his ideas that the Hebrew God YHWH was inferior to Jesus Christ, different from the God Jesus taught about, and that Jesus’ incarnation was really an illusion.
Oddly enough it is reported that Marcion was “surprised” to find himself excommunicated.
But Marcion’s ideas didn’t immediately die or fade away. The idea of a canon raised questions that the Church decided to investigate. At the time there was no canon of Hebrew Scriptures (the Council of Jamnia, though often seen as a “canonization” of the Hebrew Tanakh really was something different as canonization is a Gentile invention), and the idea that any of the Christian writings that were popular at the time were on the same level of the Hebrew Scriptures seemed preposterous to some.
The development of a canon had to invent new ideas as to how to make a “rule” for what is “Scripture” and what is not. Ideas Christians now take for granted such as inspiration, apostolic connection, etc., are not applicable to the Hebrew texts nor compatible with them. The Church had to look to its own authority and to God’s Holy Spirit to guide them in this matter.
It would take several centuries to make the rule, apply the rule, and finally come to an agreement on which books fit the rule. By the fourth century, Eusebius was able to compose a list of books which the Church at large finally agreed upon and which later the Church as a whole would “canonize.” Some of the reasons why certain books were chosen are still not clear.
It is theorized that Luke seems to have been added as a direct reply to Marcion for the heretic claimed that Luke’s writings taught that salvation was limited to an enlightened few. The writings of Luke actually offer an opposite theology and are considered the most inclusive or “Catholic” of the Gospels and historical Church accounts (Luke’s gospel is the only one not from an apostle). It is also believed that the majority of the letters of Paul were also selected over the other apostles’ to counter Marcion’s claims, but these are just theories based on circumstantial evidence.