You're not a scientist, are you? Maybe you should follow your own logic.
Christians of varying affiliations take varying amounts from the OT. Some Prebyterians, for example, think the extreme penalties dictated in the OT to be meted out on homosexuals, adulterers and even recalcitrant children should still be valid. One highly respected FReeper a while back used the example of the Midianites to indicate the fate of what will happen to proselytizing atheists like Dawkins. (To be fair to her, she did not propose that Christians themselves inflict that punishment.)
What do you mean by "a Christian nation"? I'm not trying to play word games with you; the difference is extremely vital. Is America a Christian nation in the sense that it was founded primarily by and on Christian principles? Yes. Is it Christian in the sense that the vast majority of the citizens are true biblical Christians -- that is, followers of Christ? I would venture to say no, based upon the fruit manifested in the nation.
Few if any of the founders were Biblical Christians in the sense you mean. Phillips, citing Rodney Stark, claims that no more than 15-20% of the population of the Colonies in 1776 regularly attended Church.
It was a rural population. They lived on farms and had poor transportation. But just about everybody knew the Bible.