You say you have a lot of Muslims in your family. According to Islam, Christianity is a polytheistic religion because there are three individuals who are called "God." If you want to play word games, go right ahead, but that's all you're doing.
Hong: Muslims misunderstand the concept of the trinity. Humans are 2 in one. We have a body and a spirit. I'm not suggesting we are gods though. God has a spirit and the word which became incarnate in human form: Jesus. In the Koran it is stated that Christians have 3 gods which are Mary the mother, God the father, and Jesus the son. This is a misunderstanding of Christian doctrine. Christians have never believed that Mary was a god or part of the trinity.
Katzpur: In the Book of Mormon itself, we read:
"And now, behold, my beloved brethren, this is the way; and there is none other way nor name given under heaven whereby man can be saved in the kingdom of God. And now, behold, this is the doctrine of Christ, and the only and true doctrine of the Father, and of the Son, and of the Holy Ghost, which is one God, without end." (2 Nephi 31:21).
That's probably a more straightforward statement than you'll even find in the Bible as to the unity of the Father, Son and Holy Ghost. Even in the Bible, though, Paul acknowledged the existence of beings who are "called gods" (and not only on earth but in heaven). He says:
"For though there be that are called gods, whether in heaven or in earth, (as there be gods many, and lords many,) But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him." (1 Corinthians 8:5-6)
Hong; You have changed the verse. And you need to mention the whole paragraph to understand the complete meaning too. My NIV states: "So then, about eating food sacrificed to idols: We know that an idol is nothing at all in the world and that there is no God but one. For even if there are so-called gods, whether in Heaven or on earth (as indeed there are many "gods" and many "lords"), yet for us there is but one God, the father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live." 1 Corinthians 8:4-6. The mainstream Christian view on these verses is that the other "so-called" gods that the other non-Christians believed to be real, were not real. All other gods in Biblical times were represented in the form of idols. Paul says in the verse you didn't mention that these idols were "nothing (meaning not real) and that there is no God but one". You can't just take a few verses out of the Bible without considering the context and the complete argument the writer is making.
Katzpur: We Mormons are exactly the same kind of polytheists as the Apostle Paul and C.S. Lewis were.