Only One Source of Ultimate Truth
Everybody has a set of moral rules they live by. There is no one in the world who doesn't believe that some things are right and some things are wrong. Even the hardest person, who will say until he's blue in the face, "That's fine for you, just don't tell me what to do!," will be upset if his daughter is raped. It is simply undeniable that every person believes in a code of right and wrong.
The next part is where it gets sticky. Where people get their code of right and wrong is much looser, and still looser yet is whether someone else should agree and follow a certain code of conduct, or morality. For Christians the Bible serves as a guide and source of ultimate truth, for Muslims it is the Koran, for some it is Darwinism or Freudian psychological thought, for others it is logic or feeling or 'crowd' pressures.
There must be some ultimate source of truth that trumps all others, or else we're left battling each other or compromising our values for the sake of temporal pleasures and ease of life. I believe that source of ultimate truth is found in the Bible. And by believing that I am forced to be at odds with all others who falsely, and at the risk of sounding rude, ignorantly and blindly say, "Whatever works for so and so is fine."