An absolutely a terrible idea, has this been thought through?
Who decides what "Christianity" is? In our 2000+ years of struggle we have still not answered that question.
What constitutes a "Christian"? How is it measured and what are the essential elements to make a "Christian"? Emersion? Living Water? Self confession? A particular list of denominational beliefs?
What will we do with those citizens who do not profess "Christianity"? Deportation, isolation in ghettos or camps. They will automatically become second class citizens, a short hop from abuse. Will they be allowed to vote? Maybe required to wear a patch on their coats?
Nor does this idea appear biblical. Yeshua rejected no one, feared no one, avoided no one because they had not professed faith in him. Instead he showed them His Truth. That is sufficient.
This idea is also contrary to history. During the short time there was one church, it quickly became evil and abusive and was broken up, I believe by God himself. When everyone begins to think alike He intercedes. Was this not the lesson of Babel?
This question misses an understanding of the founding, history and intent of this nation. We were founded on an unshakable faith in God however we define him. If we really have faith in God as we say, we need not exclude or restrict anyone.