First, I think it is strange that this question is arising from presidential politics, given that Mormons have a reputation of being particularly upstanding and generous citizens, and that there’s no reason in either the Constitution or traditional Christian theology to expect the President to be Christian.

But if the question is a theological one rather than a political one, several major Mormon doctrines (e.g., giving the Book of Mormon equal authority with the Bible and rejecting the Trinity, most obviously) put Mormonism outside of the theological boundaries of even the broadest definitions of the historic Christian faith.  On the other hand, Mormons themselves identify as Christian and accept Jesus Christ as the Son of God and as their Lord and Savior, which is the classic standard for membership in the Christian church, regardless of its stripe.

So, in terms of religious categorization, Mormons are probably best described as a Christian breakaway sect.  Within Christian history, they would be roughly analogous to the Nestorians, the Monophysites, or the Pelagians in the first millennium of the church: self-professed Christians who took major doctrinal stances at odds with central convictions of the traditional Church. 

There is a traditional word for these kinds of movements in Christian history, as well: heretics.  For a modern audience, declaring someone a “heretic” conjures up a lot of bad associations: burning stakes, Inquisitions, and so on.  But it actually means simply “wrong choice.”   For orthodox Christians, Mormons make some significantly wrong choices when it comes to some of the central tenets of Christian belief.

While Christian theological liberals are right to reject the oppression and persecution that generally accompanied declarations of heresy in the past, they are wrong to downplay or dismiss the significance of making a wrong theological choice. How we understand the nature and activity of God (Trinitarian thought) and how we understand God to be speaking to us (Scriptural authority) are defining theological convictions that have profound implications for Christian thought, identity, and practice, both individually and collectively.  Saying theological choices “don’t matter” is at the very least disrespectful, in this case both to Mormons and traditional Christians. 

However, Christian theological traditionalists should remember another of their central theological convictions.   Today, Protestants, Roman Catholics, and Evangelicals all affirm that “salvation comes by grace through faith.”  And faith is not the same thing as belief; faith is a way of life responding to the grace that they have recognized by following Jesus Christ as Lord in every aspect of one’s life.  To make mere belief the definition of faith is unbiblical, and to make “belief” a condition of salvation rather than “grace through faith” is itself a Christian heresy that anti-Mormon Christians are dangerously close to accepting. 

Jesus was often fond of answering a question with another question, usually one that forced the original questioner to re-evaluate his or her own beliefs or practices. So perhaps the right question here is, “How is God calling Mormons and traditional Christians to be more faithful in their own right, and how is God calling them to live and work together in building a more just and peaceful world?” Let us hope that we can all make the right choice.