There could be a difference between “as a Christian, I say...” and simply “I say.” Speaking as Christians we ought to echo the words of Christ faithfully. But we can advocate personally for any number of good and right causes, from humane treatment of animals to better sanitation to good social family values.
Of course that is correct. And it is fine for people who disagree in one area to band together to support a cause on which they DO agree. I'd be fine with signing the MD if it said something like, "while we all disagree on various aspects of the gospel of Jesus Christ, we DO agree on sanctity of life, traditional marriage and freedom of religious expression..."