Shorter Catechism Q2

What rule hath God given to direct us how we may glorify and enjoy him?
The word of God, which is contained in the scriptures of the Old and New Testaments, is the only rule to direct us how we may glorify and enjoy him.

Sometimes the descriptor "Word of God" is used to describe Jesus Christ (eg, John 1:1ff), but that is not the case in this catechism answer. In this case, the word of God is the written expression of God's message to the all people which He has given us in the Scriptures, aka the Holy Bible, both its New and Old Testaments. According to the Exposition that I am reading, the word "Testament" is a way of referring to the covenants (promises) of God that are at the core of the Biblical narrative.
In 2 Timothy 2:2, Paul exhorted Timothy to entrust Paul's apostolic teachings to those who would be reliable in their witnessing to others. One way this happened was through the collected canon of Scripture. 2 Timothy 3 also has something to say on the scriptures, but that's my Bible passage for Monday so I'll consider it then. In 1 John 1:3-4, the disciple John writes "We proclaim to you what we have seen and heard, so that you also may have fellowship with us. And our fellowship is with the Father and with his Son, Jesus Christ. We write this to make our joy complete." As I saw with the first catechism question, the end point of a relationship with God, based upon the direction ("rule") of the Bible, is joy in Him.
Peter's second letter also touches on the topic of the apostolic authority and authorship of the canon of Scripture. 2 Peter 1:12-16, 19-21 says, "So I will always remind you of these things, even though you know them and are firmly established in the truth you now have. I think that it is right to refresh your memory as long as I live in the tent of this body, because I know that I will soon put it aside, as our Lord Jesus Christ has made clear to me. And I will make every effort to see that after my departure you will always be able to remember these things.
We did not follow cleverly invented stories when we told you about the power and coming of our Lord Jesus Chris, but we were eyewitnesses of his majesty. ...
And we have the word of the prophets made more certain, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts. Above all, you must understand that no prophecy of Scripture came about by the prophet's own interpretation. For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit."
Thus the three apostles (Paul, John and Peter) who wrote the majority of the doctrinal epistles found in the New Testaments all concur as to the authoratitive nature of their accounts of Jesus Christ and the truth of the gospel based upon His atoning death.
The Westminster Larger Confession adds this reasoning, in its answer to its fourth question: "The Scriptures manifest themselves to be the Word of God, by their majesty and purity; by the consent of all the parts, and the scope of the whole, which is to give all glory to God; by their light and power to convert sinners, to comfort and build up believers unto salvation: but the Spirit of God bearing witness by and with the Scriptures in the heart of man, is alone able fully to persuade it that they are the very Word of God."