Here's a brief version of an LDS answer: Before God created this world, we were all in a pre-existant world with him. We are all his spirit children, God was/is a glorified resurrected being. We wanted to become like him. He created this world for us to receive physical bodies and to be tested. We agreed to it and eagerly anticipated the chance to progress.
Somehow the fall was necessary to create an imperfect, mortal world. But Christ's mission was already in place to redeem men from the fall. The fall brought death into the world, and Christ's atonement saved us from physical death through the resurrection and from spiritual death by paying for our sins if we repent.
So each of us can come into this world, receive our body, and prove to ourselves our own worthiness. We will all have the opportunity to hear and accept or reject the Gospel and utilize the necessary ordinances for exaltation in heaven. We are responsible to do our best, but it will never be "enough", so Christ's grace fills in where we fall short. Beyond this life there will be more progression still.