The Church (apostles and saints) wrote the New Testament, and the Church (subsequent bishops) defined its canon. Scripture should be interpreted from within the living tradition of the Church. This is not narrow and limiting, but expansive and deep. It is also reasonable, for example, a living author had written a book many years ago and had been teaching that book every day: Who could interpret that book better than he? .
Faith is a response to divine revelation. The Church did, not invent what the Church teaches, and summarizes in her creeds. It was handed down to the Church from Jesus Christ, God in the flesh. That is why it is called Sacred Tradition - "sacred" because it came from God, not mere man, and "tradition" because it was handed down. Sacred Tradition is part of "the deposit of faith," which also includes Sacred Scripture. .
It is authoritative because Christ authorized his apostles left bishops as their successors, and, with the authority Christ had given them, they ordained and authorized their successor bishops. This apostolic succession is the historical link between the Catholic Church today and Jesus Christ himself.
Man was created "in the image" of God. What is the image of God? It is not only the soul. Although God's nature is spirit, not body, yet "the human body shares in the dignity of 'the image of God': .it is the whole human person that is intended to become, in the body of Christ, a temple of the Spirit." That is why our bodies are holy, temples of the Holy Spirit. That is mostly what God asks of us.
God has revealed himself to Man in many ways, and these are just a few reasons how. Through Divine Revelation, the canon and mostly the Church and the Bible.