In John 17 Jesus prays to the Father, "Sanctify them in the truth; your word is truth" (John 17:17).  This verse is interesting because Jesus does not use the adjectives alethios or alethes ("true"), which we might have expected, to say, "Your word is true."  Rather, he uses a noun, alethia ("truth"), to say that God's Word is not simply "true," but it is truth itself.

The difference is significant, for this statement encourages us to think of the Bible not simply as being "true" in the sense that it conforms to some higher standard of truth, but rather to think of the Bible as being itself the ultimate definition of what is true and not true:  God's Word is itself truth.  Thus we are to think of the Bible as the ultimate standard of truth, the reference point by which every other claim to truthfulness is to be measured.  Those assertions that conform with Scripture are "true" while those that do not conform with Scripture are not true.

What then is truth?  Truth is what God says, and we have what God says (accurately but not exhaustively) in the Bible.

(Wayne Grudem, Systematic Theology: An Introduction to Biblical Doctrine. Published by IVP, 1994. p. 83.)