Why do you believe in moral absolutes?
The Bible makes it very clear that God sets moral laws—the standards of right and wrong. People are not given this option (Isaiah 45:19, Romans 1:25). The penalty for challenging God on this is severe in a very ultimate sense (Genesis 2:17, Isaiah 5:20, Romans 6:23, Jude 7).
But we can also intellectually defend moral absolutes over moral relativism. First, relativism is always self-contradictory. The statement that "there are no absolutes" is a self-contradictory statement, because it itself expresses an absolute. It is therefore an internal contradiction and thus it cannot be true.
To be a relativist, one must believe that (a) There is no truth, (b) Nothing is knowable, and (c) Nothing is meaningful. Yet the fact is that all people live their lives with some moral absolutes. If all life were truly relative, the cry for "fairness" would disappear. If all life were relative, there would be no definition of "abuse," or "racism," etc. The avid relativist will scream as loud as anyone else if his car is stolen!
The best example of a moral relativist—when taken to its logical conclusion—is a sociopath.
Actually, it is fairly easy for most people to see that objective moral standards do exist. We might ask you, do you think it is acceptable to burn people at the stake because we think they are witches? Or what if the Christian Coalition suddenly started rounding up homosexuals and torturing them? Of course you would rail at the thought!
If you are a student, let's say you turned in "A" work all semester and when you got your report card you see that you got an "F" in the course. Or if you are an employee, would it be right for the boss to fire you without cause? If there were no such thing as objective moral standards, you would have no right to complain in such situations. You would be obligated to accept anything and everything.
The idea of no absolutes is often expressed today with the word tolerance. Tolerance, in today's usage, proposes that all ideas are morally equivalent—that truth is relative, that everything we have learned about morality in 4,000 years of recorded human experience is now negotiable.
If tolerance is a stand-alone virtue, then we must tolerate everything. Following that logic, then, we must be tolerant of, for example, slavery or a South American drug cartel, etc. Is whoever wields the most power the most moral? Of course not. But the relativist cannot avoid ending up with that view.
We submit that tolerance and love are not the same thing. Jesus calls us to a standard higher than mere tolerance. He calls us to compassion. While Jesus reached out to prostitutes, tax collectors, and the downtrodden, he did not preach "tolerance." He helped people see truth. Jesus loved the adulteress who was going to be stoned. He loves us even though we also deserve the eternal death penalty. Jesus saves us and tells us, as the adulteress, to "leave your life of sin" (John 8:11). And his message changes the lives of people he touches forever.
Evidence also supports the view that there are moral absolutes just as there are physical absolutes. If we continually challenge the physical laws of nature, for example, driving recklessly, sooner or later it will catch up with us. In the very same way, if we regularly challenge God's moral laws, we can expect to suffer the consequences.
It is important to recognize that God did not give us moral laws, i.e. rules by which to live, because He is mean or hateful or arbitrarily restrictive. He gave us rules precisely because he loves us so much that he wants us to be safe and happy.
Again we call on evidence. There are many statistics to verify the validity of the effects of following the lifestyle espoused in the Bible. For example, studies show that monogamous married heterosexual couples report greater happiness and "overall life satisfaction" as well as greater sexual fulfillment than any other lifestyle. And people living a Christian lifestyle are significantly healthier and live significantly longer on average than other groups.
See also the next two questions: If it feels good for me isn't it right? and Why can't I live my life as an agnostic?