Unless I've read too rapidly, I haven't seen the sense that we are to bring all areas of our lives into submission to the Christ that saved us: that would include how we vote, how we run our business, our call for what is right in society, how we educate our children, and so forth. When the gospel, by God's grace, pervades churches, communities, and whole nations, radical change will occur. It starts with a radical change of heart followed by the faithful teaching of God's word and its application to all areas of life.
As others have mentioned, government, or for that matter culture, can't change the heart, so Constantine could no more make a Christian nation than I could turn a rabbit into a dog by placing it in a kennel and saying it is so. A more recent example would be the "do good for the sake of doing good" Christianity pushed by Victorian England. We are still seeing the negative results played out across the globe.
As others have mentioned, government, or for that matter culture, can't change the heart, so Constantine could no more make a Christian nation than I could turn a rabbit into a dog by placing it in a kennel and saying it is so. A more recent example would be the "do good for the sake of doing good" Christianity pushed by Victorian England. We are still seeing the negative results played out across the globe.