At least in regards to the NT, the various canons was really a list of writings approved to be read publicly in the assemblies. It got its kickstart with the heretic Marcion who developed the first canon of Paul's letters and an edited version of Luke. Once that started, bishops and local councils realized that they did need to have an official list. (Ill continue in a bit... need to head out to work)
To add to this, just for the sake of those who may be unfamiliar with this part of history:
Marcion of Sinope was a early heretic, as his name suggests he came from Sinope, a coastal city on the northern coast of what is today modern Turkey. He apparently had some wealth, as when he came to Rome and became a member of the church there he donated a large sum of money to help out the community. However he came to believe that the God of the Old Testament was not the God and Father of our Lord Jesus Christ, and began to teach that there were two gods: the old god of the Jews, and the new god never-before-heard about from Jesus.
This immediately brought problems in the Church in Rome, and a number of years after his arrival to Rome, he was formally excommunicated because he insisted on teaching his new doctrines contrary to the apostolic teaching of the Church.
In response, Marcion established a new, rival church, returning to Asia (modern Turkey), and gained followers for his split-away church. In order to teach his doctrines in his new church, Marcion produced an official canon of Scripture consisting of only ten of Paul's letters (though with some of Marcion's own edits so as to remove mention of anything pro-Old Testament), as well as a very heavily edited version of Luke's Gospel, which was presented as "The Gospel of the Lord".
The Marcionites, as they came to be called, seem to have been influenced also by the teachings of the Gnostic Valentinus of Rome, and so the Marcionite sect is usually placed as a gnostic movement due to, at least later, Gnostic teaching. And it would survive for several centuries after Marcion's death, but its peak was during the late 2nd/early 3rd centuries
It is often believed that Marcion's attempt at creating his own biblical canon for his schismatic sect was a catalyst among orthodox Christian clergy to dedicate more time and attention to what books should be read in the churches, and by the end of the second century a rather firm proto-New Testament Canon is very clearly seen, such as from the Muratorian Fragment (often dated to between the late 2nd and early 3rd centuries). In fact the only real debates over the New Testament, throughout both Antiquity and the early Medieval period, was a small group of "disputed works" (called Antilegomena in Greek), some of those disputed works eventually gained universal acceptance as canonical; some did not. Though the ones that did not gain canonical acceptance are still, nevertheless, important works that can benefit us in our study of early Christian thought. For example the Didache is an early Christian work of Christian teaching and instruction, which may have been written as early as 60 AD.
So Marcion's contribution to the Biblical Canon is quite indirect; in that the Church responded against the Marcionite sect by ensuring that what was read in the churches was what has been believed and confessed since the beginning. To hold fast and firm to the teaching which had been received from the beginning, and to safeguard it by faithful preaching and teaching. That what happened when the Faithful gathered was a faithful preaching of true Christian faith, grounded in the real and historical Jesus Christ our Lord, and His holy apostles which He sent out to preach His word to the nations, of which every bishop and presbyter could trace his calling and ordination back to by direct link with those apostles.
-CryptoLutheran
