What is the purpose of this mortal life? Three things for a Christian

1. To know and desire resurrected eternal life in Heaven, after death. 

This, with pre-mortal spirit life, is the frame for purpose in mortal life.  

2. The quest or destined-task for our life - this typically varies with age and situation.  

This is the positive purpose of mortal life; what we are trying to do. 

3. What we need to learn from experience; what life is trying to teach us. 

Learning is the negative purpose of mortal life, our response to our situation. 

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How do we discover the nature of Heaven, our quest, and life-lessons?

This has changed through human history:

1. We are told exactly what to do, by a missionary or church. Our job is to obey. 

2. We learn, read, reason what to do; selecting with discernment from church-approved sources (e.g. tradition, scripture, theology, church creed/ doctrines/ rules etc...). 

3. We do all of the above - using any or all of the above sources, and more; and our own imagination and creative endeavor; to infer what to believe and do: based ultimately on our deepest attainable intuition.