Read the Humanist Manifesto - I believe it addresses the problem of attaining morality without religion. In order to have morality, you must have a standard - who sets it? The problem is that humanists have no moral standard other than man, and each man has his own opinion on what that standard should be. Our founding fathers (Washington, Adams, Madison, et al.) stated over and over that you can't have morality without religion. Read Washington's farewell address - I'm sure you can find that on-line. While you're at it, read his inaugural address. But what did Washington know - he was only the Father of our Country. "Virtue" is meaningless without religion because it is a moving target with no standard.
Then you do not understand the heritage of America or the reasons why our government was set up the way it was. A French observer, de Toqueville, said that in America, the notions of Christianity and freedom are "intimately united" and inseparable in the minds of Americans. It has to do with the notion of self-governance (a pillar of our system) which holds that people will govern themselves and restrain themselves from evil based upon their fear of God and relationship with God. No fear of God, no restraint! Just look at out society today - it is plain to see that there is no fear of God - in fact the courts are driving God out of society while gay rights, tolerance, hedonism, in-your-face perversities of every sort are the norm. There are two ways to restrain sinful man (sinfulness of man is another cornerstone of our system) - God or the gun. See the correlation now?
Sort of like having thousands of religions, sects, and denominations, each of whom claims the one true standard, and which at some time in their history, have killed people for disagreeing with them. Of course you will argue that it is the Bible, not denomination that is the one true standard. I suppose that explains why there is so much agreement. After all, it's all in the book, and anyone can understand it.