From Chapter 1, page 2 of The Truth War, by Dr. John MacArthur:  So what is truth?  Here is a simple definition drawn from what the Bible teaches:  truth is that which is consistent with the mind, will, character, glory, and being of God.  Even more to the point:  truth is the self-expression of God.  That is the biblical meaning of truth…Because the definition of the truth flows from God, truth is theological. 

Truth is also ontological-which is a fancy way of saying it is the way things really are.  Reality is what it is because God declared it so and made it so.  Therefore God is the author, source, determiner, governor, arbiter, ultimate standard and final judge of all truth.