The Scriptures are sufficient, clear, authoritative, and necessary. I have given some quotes from great theologians of the past for the first three. I want to finish out with some quotes about the necessity of Scripture.
“Just as old or bleary-eyed men and those with weak vision, if you thrust before them a most beautiful volume, even if they recognize it to be some sort of writing, yet can scarcely construe two words, but with the aid of spectacles will begin to read distinctly; so Scripture, gathering up the otherwise confused knowledge of God in our minds, having dispersed our dullness, clearly shows us the true God…At any rate, there is no doubt that firm certainty of doctrine was engraved in their hearts, so that they were convinced and understood that what they had learned had proceeded from God. For by his Word, God rendered faith unambiguous forever, a faith that should be superior to all opinion.” (John Calvin, Institutes of the Christian Religion, I:VI:1, 2)
“We know him by two means: First, by the creation, preservation, and government of the universe, since that universe is before our eyes like a beautiful book in which all creatures, great and small, are as letters to make us ponder the invisible things of God: his eternal power and his divinity, as the apostle Paul says in Romans 1:20. All these things are enough to convict men and to leave them without excuse. Second, he makes himself known to us more openly by his holy and divine Word, as much as we need in this life, for his glory and for the salvation of his own.”(Belgic Confession, Article II)
“Although the light of nature, and the works of creation and providence, do so far manifest the goodness, wisdom and power of God, as to leave men inexcusable; yet they are not sufficient to give that knowledge of God and of his will, which is necessary unto salvation: therefore it pleased the Lord, at sundry times, and in divers manners, to reveal himself, and to declare his will unto his Church; and afterwards, for the better preserving and propagating of the truth, and for the more sure establishment and comfort of the Church against the corruption of the flesh, and the malice of Satan and of the world, to commit the same wholly unto writing; which maketh the holy scripture to be most necessary; those former ways of God revealing his will unto his people being now ceased.” (Westminster Confession of Faith I:I)
“Was it necessary for God to reveal himself by the word; or was the word of God necessary? For there were formerly (and are even to this day) various persons who believe that there is sufficient assistance in human reason to enable to live well and happily…But the orthodox church has always believed far otherwise, maintaining the revelation of the word of God to man to be absolutely and simply necessary for salvation.” (Francis Turretin, Institutes of Elenctic Theology, II:1:I, II)
“For this dispensation Scripture is not only useful and good, but also decidedly necessary for the being (esse of the church…Scripture is the only adequate means of guarding against the corruption of the spoken word and of making it the possession of all human beings. The sound of the voice passes away, but the written letter remains. The brevity of life, the unreliability of memory, the craftiness of the human heart, and a host of other dangers that threaten the purity of transmission all make the inscripturation of the spoken word absolutely necessary if it it to be preserved and propagated.” (Herman Bavinck, Reformed Dogmatics, Vol: 1 Prolegomena, p. 471)
“The written Word is, first, necessary for our relationship with God as our covenant Lord. We have seen that covenant is a relationship between the Lord and his servants or vassals, characterized by the Lord’s control, authority, and presence. In this relationship, the Lord speaks to his vassals, defining the covenant relationship…From this we see that the covenant is a verbal relationship, among other things…So if there are no written covenant words, there is no covenant, nor is there a covenant Lord.” (John Frame, Doctrine of the Word of God, p. 212, emphasis his)
“This kind of statement [he has just quoted John Calvin, see above] on Scripture argues that we must identify Scripture as the necessary Word of God because without such a Word our knowledge of God would be insufficiently grounded, unreliable and even (we might say) too subjective…What has rarely been pointed out is that not only Calvin, but also many of his theological heirs, were primarily concerned with believers’ assurance that their knowledge of God is knowledge of the true God, and not of an idol, and therefore their relationship with God was genuine and not a fantasy or a sham. This is the focus of the doctrine of the necessity of Scripture.” (Timothy Ward, Words of Life, p. 99, emphasis his)
All these quotes are from Kevin DeYoung.
“The one we need to know most cannot be discovered on our own.”
“We need Scripture to live the truly good life.”
“All truth may be God’s truth, but all saving truth is revealed truth.” (emphasis his)
“We need the revelation of God to know God, and the only sure, saving, final, perfect revelation of God is found in Scripture.” (Taking God at His Word, p, 86, 87, 88, and 90)