The American Constitution is based on a set of principles that should be guaranteed to all. So important was the perception of the need for God-given values, that some who drafted the Constitution refused to sign it until the Bill of Rights was added.
There is little scientific basis for the most basic of values. Why do all people deserve a right to life, a right to worship God, or a right to defend themselves? Those who deny that these rights are dictated by God suggest that they are only values if they benefit society as a whole. If that were true, these values could not be guaranteed to each person--only in those cases where protecting that value for that person will outweigh the costs on society.
At some point there must be faith in God's rather than man's judgment. To the extent that America determines that values should not be based on God, these values will be eroded more and more. When a society rejects God's values altogether, history has witnessed that it will fail.