Why does God leave us on earth after we are saved?
Aside from the obvious: to witness to unbelievers the salvation of the Lord. The doctrine of rewards for works after justification, in fact, does not stand alone. It is tightly bound to a well defined and grounded theodicy (the vindication of divine goodness and providence in view of the existence of real physical and moral evil) in the world. A valid theodicy must successfully answer this attack syllogism:
God is entirely good and omnibenevolent.
God is omnipotent.
Since evil exists, God is either not entirely good, or he is not omnipotent.
Or, how can an all-powerful, all good God allow evil and its devastating results to exist?
One good reason for allowing evil to exist temporarily is that it’s presence, both physically and morally, is necessary to the development of moral virtues: Bravery, kindness, mercy, patience, peaceableness, gentleness, self-control, truthfulness, compassion, humility, etc. God might have created a world without evil being possible. But in doing so a certain kind or level of goodness, but lacking the possibility of developing these virtues, would it be maximally good?
Notice how the moral virtues really dovetail with and describe the Christian virtues including the Fruit of the Spirit.
1The atheist or agnostic using the attack syllogism have an even greater problem with evil! If they acknowledge that evil exists, then they must admit that objective good and evil exist as moral categories, and the universe and human existence is not merely automatic electro-chemical reactions. For on the naturalist view, there is no such thing as real evil or good. There is only “is”. So they have to steal from and assume the truth of the theiest’s worldview to make their argument against theism.