The word Canon means “measuring stick.” Scripture is how we measure other claims to divine revelation. When someone says, “God told me,” his claim must be wrong if it contradicts what God has already spoken.
The Bible isn’t all that God has ever spoken to people, but it has already been tested. Many prophets in Jeremiah’s day said that judgment would not come, but when judgment came it was clear that Jeremiah was the true prophet.
Though the Bible does not contain all God has said, it’s the minimum that all Christians agree on. It is therefore how we can evaluate other messages that people claim are from God. Deciding on which particular books made it into the Bible, though, took a long time as God’s Spirit led the church.