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..."
Or even “we all profess a faith in Jesus Christ as savior of the world and personal savior, and that from this comes... [whatever].” Sometimes it’s not helpful to highlight the “protest” part of “Protestant.” I really admire the way C. S. Lewis, for one, brought the branches of the creed together in his books.