When I was doing my doctoral research, I was assigned a historical study on Martin Luther's attitude toward Islam, which was contained in his writings on the Turks, who, as the principal agents of The Religion of Peace were again invading Europe. In the 1500's he interpreted the Muslims' religion as a form of Arian heresy, based on its teaching that Christ was a creature and not the eternally begotten Word of God.*
He did not consider Islam as a "non-Christian religion" because he had no category for that. In his time, religions were Christian (orthodox or heretical), Jewish, or pagan. That was it. Since the Muslims were monotheists, had a scripture that tried to relate itself to the Bible, and had a doctrine of Christ, however deformed, they could not be pagans. So they were heretics.
Which reminded me of St John of Damascus, the first Christian to write a polemic against new-born Islam, which he knew intimately and personally. As the Religion of Peace rolled its conquering armies through the Christian Middle East, it took Damascus by siege in 634, a mere two years after Muhammad's death. John was born 40 years later, into a Christian family that served the Caliphate as it had served the Byzantine Emperor. John listed Islam as "the heresy of the Ishmaelites" who, as Luther later would write, held unorthodox "Arian" views of Christ as a creature.
As one of the subjects of my doctorate, John Macquarrie, learned from the Germans, you cannot underestimate the effect on thinking which derives from which questions you ask (Fragestellung) and what kinds of categories you have to create your answers (Begrifflichkeit).
Back in the day, unlike now, it could not have become a question whether Mormons were Christians or not. Different Begrifflichkeit, different Fragestellung. But that assumption, that they were, would have been made almost mute by the far more important and damning judgment that they were, like the Muslims and the Cathars, heretics.
*Unlike a pious and angry lady who once broke down in tears when --in a classroom setting, I must add-- I corrected her statement that in the Gospel of John, "Jesus created the world", the orthodox tradition differentiates between the Person of Christ as the Second of the Trinity and his human nature, his human body and human soul, intellect and will, created and born in time of the Virgin Mary.