It is very important to know that Christianity and Mormonism are not cut from the same cloth and believe very different things about the Gospel and one of the aspects of Mormonism is polytheism, something that indeed separates it from true Biblical Christianity, ONE of many things.
The Bible makes it abundantly clear over and over that there are NO other Gods besides YHWH. Here are many of the texts affirming this:
You were shown these things so that you might know that the LORD is God; besides him there is no other."
"See now that I myself am he! There is no god besides me. I put to death and I bring to life, I have wounded and I will heal, and no one can deliver out of my hand."
For who is God besides the LORD? And who is the Rock except our God?"
"You are my witnesses," declares the LORD, "and my servant whom I have chosen, so that you may know and believe me and understand that I am he. Before me no god was formed, nor will there be one after me."
"This is what the LORD says-- Israel's King and Redeemer, the LORD Almighty: I am the first and I am the last; apart from me there is no God."
Do not tremble, do not be afraid. Did I not proclaim this and foretell it long ago? You are my witnesses. Is there any God besides me? No, there is no other Rock; I know not one.""
so that from the rising of the sun to the place of its setting people may know there is none besides me. I am the LORD, and there is no other."
For this is what the LORD says-- he who created the heavens, he is God; he who fashioned and made the earth, he founded it; he did not create it to be empty, but formed it to be inhabited-- he says: "I am the LORD, and there is no other."
Declare what is to be, present it-- let them take counsel together. Who foretold this long ago, who declared it from the distant past? Was it not I, the LORD? And there is no God apart from me, a righteous God and a Savior; there is none but me."
Remember the former things, those of long ago; I am God, and there is no other; I am God, and there is none like me."
Then you will know that I am in Israel, that I am the LORD your God, and that there is no other; never again will my people be shamed."
It is very clear from the scripture that there are NO other Gods. What you have are emphatic statements.
Now here are some possible objections to respond to:
"Oh but it means the only God of this planet in those contexts?"
The statements don't imply such a thing, In fact it demonstrates that there is only ONE, period. Other gods are called demons in the scriptures, both the Old Testament and the New Testaments:
17 They sacrificed to false gods, which are not God—
gods they had not known,
gods that recently appeared,
gods your ancestors did not fear."
"Psalm 106:37 They sacrificed their sons
and their daughters to false gods."
20 The rest of mankind who were not killed by these plagues still did not repent of the work of their hands; they did not stop worshiping demons, and idols of gold, silver, bronze, stone and wood—idols that cannot see or hear or walk."
Several more texts could be cited to bring home the point that the other gods of the nations or even the hypothetical scenario that there are supposed gods on other worlds, they would all be worthless idols or even demonic entities. Although the Mormons do not worship these entities, To say that there are other gods that exist, is certainly not a biblical thing to say.
The Old Testament even goes as far as telling us not to invoke deities in and oath or even, not uttering the names of false gods on your lips. This is quite serious.
"What about 1 Corinthians 8:6?"
"1 Corinthians 8:4 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.” 5 For even if there are so-called gods, whether in heaven or on earth (as indeed there are many “gods” and many “lords”), 6 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."
The text is not saying that polytheism is acceptable or is even something Paul acknowledges. What is actually being talked about is that while the world claims that there are many gods, Paul and the Christians affirm and know that there is only ONE God and ONE Lord. See my comments on 1 Corinthians 8:6 here in this article:
"What about Psalm 82:6?"
The text in question doesn't teach that men can become Gods or that they are Gods. As I have said before, What Jesus is using in the context is an a fortiori argument or an argument from a yet stronger reason to further demonstrate his deity, which most anti-Trinitarians overlook. Keith Thompson, CEO of Reformed Apologetics Ministries explains this point better than I do in his documentary "Word of Faith Teachers: Origins & Errors of Their Teaching" which can be found here (https://www.youtube.com/watch?v=6KIA06kDRw4 Watch from position 56:51-1:00:28).
Watch the section in question if you want to understand what the text itself means.
"If we are polytheists, What about you? don't you believe in the Trinity?"
The Trinity is not polytheistic at all. It teaches that in the very being or essence of God, there exist Three Distinct persons, Not three beings in one being or three persons in one person . There are not three separate Gods, but one Eternal God which the TANAKH and the NT make very clear. Hence the Trinity isn't polytheistic in the slightest.
"Why don't you pray like in James 1:5 to see if the Book of Mormon is true?"
Let's see what James 1:5 saying:
"James 1:2 Consider it pure joy, my brothers and sisters,[a] whenever you face trials of many kinds, 3 because you know that the testing of your faith produces perseverance. 4 Let perseverance finish its work so that you may be mature and complete, not lacking anything. 5 If any of you lacks wisdom, you should ask God, who gives generously to all without finding fault, and it will be given to you. 6 But when you ask, you must believe and not doubt, because the one who doubts is like a wave of the sea, blown and tossed by the wind. 7 That person should not expect to receive anything from the Lord. 8 Such a person is double-minded and unstable in all they do."
The context has nothing to do with asking if a certain book is true, what it is talking about is if you are uncertain about something and you need the wisdom, God will grant it if you ask. This text cannot be used to prove the book of Mormon or other.
(I have asked God if the Book of Mormon is true and he said NO if any one is curious.).
"I feel in my hear that Mormonism is true"
A feeling in the heart doesn't make something true, in fact, the heart is deceitful above all things (Jeremiah 17:9). Our hearts must never be used as the criteria of truth, many can make this claim that they believe that their religion is true because their heart says so. See the following paper: http://answering-judaism.blogspot.co.uk/2014/06/spiritual-experiences-what-can-they.html
"You need the Book of Mormon to understand the Bible"
Says who, Some fellow who never knew the apostles directly but claimed revelation? I am sorry, The Bible is what is to be used as a witness for or against the book of Mormon, you don't need the book of Mormon to supplement the Bible, nor can it supplement the Bible.
More objections may be looked into if the Lord Wills