While the New testament is clearly evangelical, at no point does it say that all humanity should act as Christians without being Christian. Like the Jews, Christians are held to Christian rules, and saved by them. They congregate with each other to get their church on.
But you don't become a Christian by living in a nation that outlaws contraception or makes it a luxury good, by living in a country that outlaws medically necessary procedures like pregnancy control or Blood Transfusions (What, Christian Scientists and Seventh Day Adventists aren't Christian?), by living in country where sexual acts between consenting adults are illegal, liquor sales on your sabbath are illegal or slavery is illegal (yes, the same immoral religion was used to justify that 200 years ago).
I am absolutely fine with Christians living in America and practicing their faith, including evangelizing and sharing the good news to bring more into the fold of Christ's Love. What I am not fine with is immoral Christians thinking that if we make America into a regualtory cartoon of Christianity than all Americans become Christians. It doesn't work that way. And immoral "Christians" are going against God's word for thinking it does.
Render unto Caesar what is Caesar's: equal protection under the law. The constitution is short and to the point because it doesn't spend any time squaring itself with bible verses. I say we take the hint. I'm a strict constructionist like that.