National Review's Kathryn Jean Lopez, in the context of Mitt Romney's Mormonism and the view of some evangelicals that it's not Christian:
I remember the first time it was explained to me at a Concerned Women for America conference that I am not a Christian because I am Catholic. At the time I wondered why we — my pro-life, conservative CWA friends and I — couldn't just strengthen our political alliance on issues we agreed on and go our own ways Sunday morning (or whenever).
This is not just getting religion out of politics: it's agreeing to the claim that the essence of your religion is a lie for the sake of a political cause. If faith is allowed to drive your political beliefs, shouldn't it drive who you politically associate with?