This belief is quite unique to Christianity; I'm not aware of any other Christian denomination promoting plurality of Gods. But at one time, missionaries did not routinely present it during their discussions with investigators. Consequently, many people who joined the LDS Church did not learn of this belief until AFTER they joined the Church. While most accepted it, a few reacted badly, ultimately left the Church, and even cited this belief as a primary motive for their apostasy.
Consequently, it is important to me that any prospective convert know of this belief and understand it in advance before deciding to join the Church. But using the Book of Mormon, the Doctrine & Covenants, and the Pearl of Great Price to convince the average person may not work, since Mormons are the only Christians who consider these works to be authoritative. So we must go to the Bible, which is the common scriptural standard of all Christians, to determine if there is Biblical corroboration of the plurality of gods.
And behold, there is plenty of corroboration. Below are listed a number of Biblical verses, according to the King James Version, which clearly proclaim the plurality of Gods:
30 I and my Father are one.
31 Then the Jews took up stones again to stone him.
32 Jesus answered them, Many good works have I shewed you from my Father; for which of those works do ye stone me?
33 The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God.
34 Jesus answered them, Is it not written in your law, I said, Ye are gods?
35 If he called them gods, unto whom the word of God came, and the scripture cannot be broken;
36 Say ye of him, whom the Father hath sanctified, and sent into the world, Thou blasphemest; because I said, I am the Son of God?
Of course, in this context, the Jews whom he addressed were obviously not gods at the time, but what Jesus was acknowledging is that, as humans, they were embryonic gods in training. They'd taken on spirit bodies, maxed out their possibilities in the spirit world, then came down here to overlay their spirit bodies with physical bodies for the critical next step. This is because our Heavenly Father, like all other gods, is a resurrected celestial being with a celestialized physical body seamlessly integrated with a spirit body. Consequently, to continue our eternal progress, we likewise had to take on a physical body in a mortal environment just like the Father once did.
Now here are other Biblical verses:
Deuteronomy 10:17: “For the Lord your God is God of gods, and Lord of lords, a great God, a mighty, and a terrible, . . .”
Psalms 136:2: “O give thanks unto the God of gods: for his mercy endureth for ever.”
Revelation 17:14: “These shall make war with the Lamb, and the Lamb shall overcome them: for he is Lord of lords, and King of kings: . . .”
1 Corinthians 8: 4–6: “As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one. 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.”
So there is ample Biblical corroboration of plurality of gods. If there was only one, why would there be repeated references to "god of gods" and "lord or lords". Of course, to allay any confusion, I Corinthians resolves it by proclaiming that, although there are many gods, we only worship one.
And Matthew 6:9 tells us which one to worship, and gives us a sample format on how to talk to Him.
Of course, when many mainstream Christians are presented with this information, they start sputtering and saying that the Bible doesn't "mean what it says", even if in the previous breath, they proclaim the Bible to be the "inerrant Word of God". They want to set aside these "inerrant" verses from their "inerrant" Bible because it doesn't corroborate their personal theology. But if the Bible is so "inerrant", doesn't it mean that ALL of it is inerrant? Even if you believe the Bible is merely authoritative rather than inerrant, doesn't that mean that ALL of it is authoritative?
Yes, we Mormons freely proclaim that many of the plain and precious truths of the Bible to have been lost, and the Eighth Article of Faith in part states, "We believe the Bible to be the word of God so far as it is translated correctly,..." , but we have adopted the King James Version as our standard for the time being, recognizing that it is merely authoritative rather than inerrant. If one considers scripture which was recorded by imperfect human beings to be inerrant, one is ascribing perfection to an imperfect product of imperfect human beings. Imperfection cannot produce perfection. Consequently, we prefer to describe the scriptures we use as "authoritative" rather than "inerrant", which acknowledges their imperfection but also proclaims their authority. We worship the Lawgiver, not the Law.
Of course, I anticipate that some wanker might now be tempted to ask "Who is our Heavenly Father's God"? The answer is that we don't know and haven't been told. No one who has ever walked the face of the earth, no matter how holy, has been given this information. Why? Because we don't need to know it. It is not an essential doctrine of salvation. We are given only the information we need to know to begin the journey back to heaven and to our Father's realms. Besides, Matthew 6:9 tells us that we only worship OUR God, so why would we need to know the specific identitites of any other gods? It is sufficient simply to know that there are multitudes of gods in the heavenlies and leave it at that.
By the way, the doctrine of "plurality of gods" is not one of those "plain and precious truths" which were lost. It's been sitting in your Bible all this time. Perhaps you ought to be asking your pastor or priest why he has not been preaching it.