In Hebrews 6, the author of Hebrews recounts God swearing an oath to confirm his promises to Abraham.
Now when people swear, they do so by appealing to some sort of authority higher than themselves in order to validate their promise. But what happens when God swears to confirm his promise?
For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself (Heb 6:13).
When God swears, he swears by his own authority, because there is no possible higher authority. There is no authority higher or equal to God himself to which he can appeal. He is the highest.
And so, by extension, God’s Word is the highest authority. No authority can possibly usurp it. The scriptures stand alone as our supreme authority.
Kirk E. Miller 