Isn't it narrow-minded for Christians to think that only they have the truth? Isn't all truth relative?

Christians claim that God is the source of all truth. And since the Bible is the inspired, authoritative Word of God, we must check our view of reality against it.

Jesus said: "I am the way, the truth, and the life: no man cometh unto the Father, but by me" (John 14:6, KJV). If that sounds narrow, it's because truth is narrow. For example, it's still true that 1 + 1 = 2. It does not equal 1.9 or 2.1, etc. It equals 2.

By definition, "truth" is what corresponds to reality and is true for all people, at all times, in all places. While our understanding or perception of truth may change with time, truth itself never changes. For example, even in the year 1500 it was true that electricity would be used to power the typical pc in the year 2011. But of course in order for that truth to be realized: 1) electricity had to be discovered and harnessed, 2) the pc had to be invented, and 3) the year 2011 had to arrive.

As for the popular notion that "all truth is relative," that is a self-defeating proposition since it is making at least one absolute claim regarding truth.