A popular saying or belief amongst skeptics of the Christian faith is, "All truth is relative." With this statement many skeptics seek to justify, in their own minds, that there are no absolute truths, including those truth claims put forth in the Bible. Yet, when the skeptic says, "All truth is relative" they are, perhaps unknowingly, making a claim that they believe is absolutely true. If what they are saying ("All truth is relative.") is absolutely true, then not all things are relative and the statement that "All truth is relative" is false.
Another closely related and popular sentiment in our society today is, "There are no absolute truths." Yet, this statement, "There are no absolute truths" is an absolute statement which the skeptic claims is true. If their statement is true, it is an absolute truth and there "There are no absolute truths" is false. Both statements ("All truth is relative" and "There are no absolute truths") are self defeating. If you have the opportunity to talk to someone who thinks this way, point out to them that the logical end of their beliefs is that they cannot believe anything at all, including that there are no absolute truths. Nothing can be true for the relativist, including relativism.