One of Mormonism's more peculiar beliefs is that human beings had a pre-existence in Heaven. God the Father has several wives, with whom He conceived our spirits. Our spirits then came to Earth to be born in physical bodies.
Biblically speaking, we know this is impossible. 1 Corinthians 15:46-47 makes it perfectly clear that the natural precedes the spiritual, specifically indicating that the first man (i.e. Adam) was of the Earth, made of dust (after which time Genesis 2:7 tells us God breathed life into him and he became a living being), but the second Man (i.e. Jesus) is the Lord from Heaven. In other words, the only human being who has ever existed before He was conceived is Jesus. Everyone else received life when they were conceived.
By writing this, God used Paul to completely invalidate Mormonism's pre-existence theory, but this doesn't stop them from abusing scriptures to try to suggest otherwise. One of those verses is Ecclesiastes 12:7, which in the KJV reads "Then shall the dust return to the earth as it was: and the spirit shall return unto God who gave it." Mormons attempt to argue that when the verse says "the spirit shall return unto God who gave it", this is speaking of some kind of return to God's dwelling place from whence it came.
There are several problems with this interpretation, not least of which is that the Jews to whom this oracle was entrusted never held to any kind of pre-existence theory. Coincidentally, it has been suggested they never had a concept of an afterlife either, as Ecclesiastes 9:5 says "For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten."
Given that Mormons believe in an afterlife, they would obviously object to any attempt to claim that Ecclesiastes 9:5 says there is no afterlife. On the same grounds, we can reject the Mormon interpretation of Ecclesiastes 12:7, especially since 9:5 is actually easier to twist against an afterlife than 12:7 is to twist in favour of pre-existence.
The answer to both misinterpretations is fairly simple. Ecclesiastes as a whole is a very geocentric book. The phrase "under the sun" is repeated 27 times in its 12 short chapters, making it perfectly clear that the physical world we inhabit is very much in view here. Heaven is not the focus. It is perfectly safe to say that the dead under the sun know nothing, just as it is fair to say that the dust shall return to the earth under the sun.
But what does it actually mean that the spirit shall return to God who gave it? This is not as complex as it might seem. Put simply, it means God gives us life. The breath of life and the spirit of God are seen as synonymous. In Job 27:3, for example, we read "All the while my breath is in me, and the spirit of God is in my nostrils;". This is Job's way of saying "as long as I live". Thus, when Ecclesiastes says "the spirit will return to God who gave it", it is an entirely natural interpretation to say that this refers to the spirit of God that gives a man life. When God's spirit returns to Him, man dies. No more life.
As with all Bible verses, the context of Ecclesiastes 12:7 matters. The Bible as a whole militates against pre-existence theory, and in context, Ecclesiastes 12:7 is no exception. Just as a contextual reading of Ecclesiastes 9:5 does not preclude an afterlife, so also does a contextual reading of Ecclesiastes 12:7 fail to establish a pre-life. Mormonism is a heretical organisation devoid of spiritual truth.
The truth is, we were all born of flesh. But in order to receive eternal life, we must be born again of the spirit. It is essential to repent of sin and believe in the Lord Jesus Christ, who is not our biological brother from one of several spirit mothers. He is God Himself, reaching down from Heaven to save those who would otherwise never see it. Repent and be saved.