The Mormons hold to a doctrine of the pre-existence of the soul. That is, that the soul of each human being existed first in the spiritual realm.
In their own words, "Before we were born on the earth, we lived in the presence of our
Heavenly Father as His spirit children. In this premortal existence, we
attended a council with Heavenly Father’s other spirit children." They claim that this doctrine is taught in Ecclesiastes 12:7: "
[After death] the dust returns to the earth as it was, and the spirit returns to God who gave it." That is, Mormons claim, if it is
returning to God, then it must have been with Him
before it was incarnated in a body. Note that this isn't a form of
reincarnation, since the Mormons do not claim that the soul existed in
another body before its present incarnation.
However, as is their wont, the Mormons are being very selective in their use of Bible proofs. Another thing that God says on this matter can be found in Zechariah 12:1: "
Thus declares the LORD, who stretched out the heavens and founded the earth and formed the spirit of man within him..." Here, the prophet clearly described the contemporaneous
creation of, and
incarnation of, a man's soul. So, when a Mormon asks where the soul was
before its current incarnation, the answer is that it wasn't
anywhere, any more than the body was somewhere before its conception.
In Ecclesiastes, Solomon is describing a circumstance analogous to ordering a pizza. When you order the pizza, it doesn't yet exist. Rather, it is prepared and then given to you. If you then return the pizza, let's say for being burnt, you do indeed give it back, because
it existed before you returned it, not because it existed
before you ordered it. In the same way, God creates the soul within the new body, apparently at conception (see Psalm 51:5), and it then returns to its Maker upon death. There is no time of spiritual existence prior to incarnation.