Then how can we trust their judgment on the what should be included within the canon of Scriptures?
"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)
It's not really their judgment. The councils and popes involved in determining the canon of Scripture were acting as the human agents for the Holy Spirit. The canon is guaranteed in the same type of way Scripture itself was written: through the human agency of men wielding a pen, inspired and guided by the Holy Spirit.
We are an incarnational Church, embodied spirits surrounded by matter. Just as Jesus used the physical realm to impart His revelation via physical objects (Himself, as a living human body; mud; spittle; water, wine, the wood of the cross, etc.), so, too, He continued the process to some degree after His ascension. Human beings were left in charge as the visible heads of the Church. But revelation and its guarantees of authenticity were still directly "of God" via the Holy Spirit. Men did not merely make things up as they went along. Sometimes they had direct revelation, as in Acts 10; sometimes they had to feel their way along, as in Acts 15, but the Spirit guided them in either case, mere material creatures though they were.
The Spirit used human beings, empowered by the laying on of hands as per Christ, through inspiration and infallibility, as the physical agents of truth. The canon was determined by the lowly material human beings attending certain councils and ratifying them, and they determined it NOT by themselves, but with the guidance of God. The Bible did not fall to earth from heaven ready-made (in Hebrew, Greek or the KJV), but had to be written, compiled, vetted, canonized and interpreted as Scripture by human agents of the Holy Spirit. The Catholic Church knows where the Bible truly came from. It's her own book. Those who are so familiar with the Fathers that they can rummage through the entire 37-volume set looking for rare and obscure exceptions to the Sensus Catholicus of the early Church, seem to be acting disingenuously when they plead that they cannot understand how the Church got the Bible. They only compound things when they additionally claim that the deuterocanonicals deserve to be removed from the canon when they cannot cite any reason, without acknowledging the authority of incarnate, human members of the Church acting as representatives of Christ, why there is a canon of Scripture at all.