The Scriptures are Authoritative

"Let this be a firm principle: No other word is to be held as the Word of God, and given place as such in the church, than what is contained first in the Law and the Prophets, then in the writings of the apostles; and the only authorized way of teaching in the church is by the prescription and standard of his Word." (John Calvin, Institutes, IV:VII:8)

"We believe that this Holy Scripture contains the will of God completely and that everything one must believe to be saved is sufficiently taught in it. For since the entire manner of service which God requires of us is described in it at great length, no one-- even an apostle or an angel from heaven, as Paul says- ought to teach other than what the Holy Scriptures have already taught us. For since it is forbidden to add to or subtract from the Word of God, this plainly demonstrates that the teaching is perfect and complete in all respects.  Therefore we must not consider human writings-- no matter how holy their authors may have been-- equal to the divine writings; nor may we put custom, nor the majority, nor age, nor the passage of time or persons, nor councils, decrees, or official decisions above the truth of God, for truth is above everything else. For all human beings are liars by nature and more vain than vanity itself. Therefore we reject with all our hearts everything that does not agree with this infallible rule, as we are taught to do by the apostles when they say, "Test the spirits to see if they are of God," and also, "If anyone comes to you and does not bring this teaching, do not receive him into your house." (Belgic Confession, Article 7, emphasis mine)

"The supreme Judge, by which all controversies of religion are to be determined, and all decrees of councils, opinion of the ancient writers, doctrines of men, and private spirits, are to be examined, and in whose sentence we are to rest can be no other than the Holy Spirit speaking in Scripture." (Westminster Confession of Faith I.X)

"The authority of the Scriptures depends on their origin. Just because they are from God, they must be authentic and divine...The authority of Scripture is nothing else than the right and dignity of the sacred books, on account of which they are most worthy of faith with regard to those things which they propose to be believed and of our obedience in those things which they command us to omit or to do. (Francis Turretin, Institutes of Elenctic Theology, II:4:1, 3)

"As the word of God it stands on a level high above all human authority in state and society, science and art. Before it, all else must yield. For people must obey God rather than other people. All other [human] authority is restricted to its own circle and applies only to its own area. But the authority of Scripture extends to the whole person and over all humankind. It is above the intellect and the will, the heart and the conscience, and cannot be compared with any other authority. Its authority, being divine, is absolute. It is entitled to be believed and obeyed by everyone at all times. In majesty  it far transcends all other powers. But, in order to gain recognition and dominion, it asks for no one's assistance. It does not need the strong arm of the government. It does not need the support of the church and does not conscript anyone's sword and inquisition. It does not desire to rule by coercion and violence, but seeks free and willing recognition. For that reason it brings its own recognition by the working of the Holy Spirit. Scripture guards its own authority. (Herman Bavinck, Reformed Dogmatics, Vol 1, p. 465)

John Frame's mentions five points in relation to the authority of Scripture. These are all direct quotes.
1. God is the author of the whole Biblical canon.
2. That we live by all of it.
3. That God has the right to speak to us about anything at all.
4. That the purpose of Scripture is redemptive in a broad sense, not a narrow sense.
5. That the redemptive purpose of Scripture is so broad that no area of human life is excluded from its concern.
(John Frame, Doctrine of the Word of God, p. 166)

The doctrine of the authority of Scripture "states we believe all of this true, that the sovereign God has indeed authored Scripture this way and chooses to relate to us in this way through Scripture. It commits us to giving the Bible the sovereign place in our lives that must follow from its central place in relation to God and his actions. (Timothy Ward, Words of Life, p. 129)

"The Bible is never wrong on what it affirms and must never be marginalized as anything less than the last word on everything it teaches." (Kevin DeYoung, Taking God at his Word, p. 78)