"Establishing" the canon is simply recognizing what is Scripture.I don't think the word 'establishing' is appropriate, by way of suggesting an act of the church, whereby one day some books supposedly became the Word of God.
Rather, if the Word of God was there in its completeness, the church basically recognized what was already there, what was already authoritative.
Tertullian wrote helpfully about this. He refered to the canon of Scripture as the 'instrumentum', the instrument by which the church defined itself, rather than the other way round.
I agree that they did not become Scripture by an act of the church. But they were recognized as Scripture by an act of the church (and likewise other books were recognized as not being Scripture). And without the recognition of what is (and is not) Scripture, how do we really possess Scripture?
Source for the Tertullian quote please? The church defines itself by its head, Jesus Christ. And since the church existed at least since Pentecost (prior to any NT Scriptures being authored), I find it difficult to conclude that the church defined itself by what was not yet in existence.