(For those picking up here, you may want to read the overview first.)
There is very little in the Bible that mentions the existence of people as individuals before we were born. It is primarily through the LDS scriptures and prophets that we know of our existence as spirits before we were born, or as the church curriculum calls it, the “pre-mortal existence.” It’s an important point in LDS theology, and is key in understanding the Mormon perspective of the world.
Most of the references in the Bible to a pre-mortal existence are concerning Christ. So, it is easy to see how most other Christian religions would consider him an exception to the rule, and therefore understandable that most Christian religions don’t speak of any existence before our birth. However, despite the lack of detail in the Bible, there are several references that should make it clear that our spirits lived with God before being sent to Earth. One is in Jeremiah 1:5 when the Lord said to Jeremiah, “Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet unto the nations.” Similarly, other individuals were also “foreordained” to their callings: Jesus, of course (1 Peter 1:20) and John the Baptist (Luke 1:17), for instance.
As the Bible provides only hints, it’s the LDS scriptures and modern prophets that have expressly defined the doctrine. What follows is my own retelling, informed by various specific sources and a lifetime in the church.
We believe that human beings are literally the spiritual children of God, which is why He loves us and why we call Him our “Father in Heaven,” or “Heavenly Father.” We lived with him as spirits before human beings ever set foot upon the Earth. Jesus Christ was our elder brother in that pre-mortal existence. He too, was only a spirit at that time. God, the Father, on the other hand, had a body of flesh and bone. (Interestingly, in none of our scriptures does it reference heavenly beings as having blood.)
As children of God, we learned from Him and progressed in our development. However, we were limited in our progression somewhat by our lack of a physical body, and, in a way, because we were with God. One of the critical elements for progression and development is agency. We needed to be able to make our own decisions and judgements without His pervasive influence. To give us the opportunity to gain physical bodies and exercise our agency, God presented a plan. If we followed His plan and were faithful to the knowledge given to us, we would be able to become like our Father.
Everything worthwhile is challenging. All would sin in this life, and in that state, we would be unworthy to return to God’s presence. So, he provided a Savior to provide an Atonement and provide a path back to Him. Jesus accepted the responsibility to be our Savior and to show us the way.
Lucifer thought he had a better idea. He proposed that we not be given our agency; he would make sure all people did what was needed to return to God. For his role, he thought he should also get the glory. He led others to rebel against the plan, and for this great sin of rebellion, he and his followers were cast from God’s presence. They would not be able to participate – at least in terms of gaining a body.
The Earth was created, and God sent His children to the families where they would have the opportunities they needed to progress as part of the plan. He foreordained prophets and other leaders to accomplish His work. We all had different talents and strengths that God considered when sending us to this life. His goal was to provide us, His children, with the best opportunities he could provide to help us learn the things that are most important and to develop godlike characteristics.