Jesus claimed to be the truth (John 14:6), and that the word of God is truth (John 17:17). He also said that we could know the truth through his word (John 8:31-32) and that we would be judged by this truth (John 12:48). Can we believe the claims? How do we know for sure that his claims of truth are true?
I think the most straightforward way to evaluate the claim, is to look at the substantiation. Many have made the claim of truth, but only one has backed up this claim irrefutably. Jesus performed many miraculous signs to furnish proof of his claims (John 10:37-38, John 20:31-32, Acts 2:22). We need to look no further than the immutable fact of the resurrection, wherein God furnished proof to all men by raising him from the dead (Acts 17:31), to see that he had the right to make such claims. Further, you may want to read more about prophecy to see the truly amazing nature of God's word. The Bible's claim to be the absolute word of God, the truth, and without error, is confirmed by the amazing prophecies set forth in its pages.
It is important to note that relativism in truth stems from a philosophy of naturalism. If we simply accept creation and the Creator this concept is not at issue. Accepting and trusting in an all-knowing, all-powerful God puts the absolutes of good and bad, right and wrong, beyond question.
Finally, it would be wise to consider the consequences of denying truth. One problem with relativism is that it incites us to begin playing semantic games to justify our position. We know that 2+2=4, and we know that the killing in Littleton, Colorado (1999) was wrong. Let's not play games with the truth in order to justify our position of relativism. The fact is 2+2 will always be 4, even if we believe otherwise.
Besides the confounding of the human language, making it vague and ambiguous, denying the ability to know truth has caused many to set aside their ability to know anything. They say, "If I can't know everything, then I can't know anything." Nothing could be more wrong. Just because I don't understand advanced calculus (and I don't), doesn't mean I don't understand basic arithmetic. We can know some things, and we can understand them with a great deal of certainty. Does this mean that we can ever know all there is to know about God and His word? No, the nature of His infinite wisdom means we will never stop learning, yet we can certainly understand the most fundamental concepts and those things necessary for salvation and Christian living.
Finally, not accepting absolute truth has some eternal consequences. Paul warns the Thessalonians about denying the love of the truth.
"And with all the deception of wickedness for those who perish, because they did not receive the love of the truth so as to be saved. (2 Thessalonians 2:10)