"Where do you think the Bible came from?"
2 Peter 1:21
For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.
"We know it's God's Word because the Church said so"
Did the Catholic Church confer on the Scriptures any authority they did not already intrinsically have by virtue of their being produced by the Holy Spirit?
"and it had the teaching authority to make that declaration,"
Did John the Baptist have the authority to decided who was going to be the Messiah?
The Holy Spirit was sent by Christ to guard and preserve the Church, and from that inspiration, the Church produced the inspired Word of God in the New Testament. The Church is also charged with preserving and teaching this deposit of faith.