The basis of our legal system comes from Roman law, by way of Norman law, English common-law etc. Our legal system is a fusion of a lot of things. If you'd gone to law school, you would have learned about the basis of our legal system.
Yes, many laws today are not based on Christian morality - they are based on godless humanism which leads to the erosion of freedom and slavery
Most laws today are morality-neutral. Trying to use the Bible as a guide to what laws to pass is as foolish as using the Koran. The vast majority of laws simply do not implicate the Bible in any way. Try finding a biblical justification for our system of riparian water rights, or zoning law.
Have you gone to law school? If you have, then I have some questions for you. Why were our laws once derived from divine law (Blackstone), but later replaced with legal positivism (Oliver W. Holmes and Charles Evans Hughes)? Do you know what legal positivism is?