The Bible is the cradle wherein Christ is laid. – Martin Luther
If you’re ever at a loss of words when trying to articulate any given theological topic, I find the best place to turn is to the historic Reformed Confessions. The men who penned the words in the Confessions agonized for countless hours over how to communicate the truths found in Holy Scripture in the most accurate and concise way. That’s why, when I look to where I can find the best statement on the authority of Scripture, the Westminster Confession of Faith is the first place I turn. Here is Chapter 1, Section 4
The authority of the Holy Scripture, for which it ought to be believed, and obeyed, depends not upon the testimony of any man, or Church; but wholly upon God (who is truth itself) the author thereof: and therefore it is to be received, because it is the Word of God.
The authority of Scripture does not depend on any man, nor does the Bible get its authority from the Church, but instead its authority comes from the true fountainhead of truth and authority: The Living God, the One who authored it. This is why we believe the Bible, not because it is beneficial to follow the instructions contained therein (although it is,) nor because our tradition says we should believe it, but because it is the breathed out Word of God.
But how do we know the Bible is the Word of God? Easy enough answer: because it claims to be the Word of God. Before you say “that’s circular reasoning,” let me ask you this. If you authenticate X with Y, which is the higher authority? Y is, because it is what you used to grant the authenticity of X. If we truly believe the Bible is the highest authority, we cannot use external sources to authenticate it, otherwise what we used as an authenticating tool has surpassed the authority of Scripture.
Now, that sounds nice, but how does it work out practically?
The Bible is true because God is the standard of truth, and because God testifies to us through the Bible. We know that God is perfect (Psalm 18:30), never lies (Titus 1:2), and that He breathed out Scripture (2 Timothy 3:16) thus there are no lies in Scripture. It is entirely true.
The Bible is true because of the impossibility of the contrary. Our moral standard, the laws of nature, and the existence of the universe itself all point to the one Living God. If the Bible were in error about the nature of God and the world, God would be made a liar, and the world would succumb to chaos.
The truth of Scripture is the only lens through which the world we live in makes sense.
The condition of man. The nature of the world. The testimony of God contained therein. All of these irrefutably point to the Holy Bible as the absolute truth given to man from God. So not only is the Bible reliable, it is essentially and necessarily so.