"Then how can we trust their judgment on the what should be included within the canon of Scriptures?"
How could the disciple trust the witness of John the Baptist?
(Luke 7:35)
John the Baptist was just renewing the message of the Old Covenant by calling the people to repent of their sins and return to the Covenant. With the person of Jesus Christ we have something new. His message that he was the Christ and the Son of God was validated by the Resurrection. The witness of the Apostles to the Resurrection was the testimony about about a factual event. The selection of the canon of the Scripture on the other hand is a question of judgment and authority. As a Catholic I believe that the visible Church has been given the charism of Infallibility. It is with this that I can have confidence in her judgment regarding the selection of Scripture. If we are to reject this and say that the early Church was made up of only fallible men then their selection of the canon must also be fallible. If so, then how can we have confidence that what we call the Word of God truly is? Thus the authority of the Bible itself becomes subject to private judgment.