There is one factor about God that most Christians miss and it is so obvious to me that I can't see how they are missing it. God created us in "His own image". Is God a human being? I am sure you will agree that He is not! So we can't be either otherwise we wouldn't be in "His own image"! God is pure spirit or soul (or whatever words you might choose to use), but most certainly He is not human. So when He created us in His own image, He created us as spirit/soul as well. And he gave us free will. And with that free will we decide when we will come to earth to learn the lessons we need to learn.

There are also many souls who have never come to earth. It takes a lot of courage to choose to come from a place of absolute love and peace into the harsh reality of duality, the human existence. And many souls have not developed that courage yet. (This is also why there are miscarriages and stillborn babies - the soul changes its mind and goes back - only to try again some other time when they work up some more courage.) But they do have to eventually or they won't realise their full spiritual potential.

God creates souls and each soul can come to earth in different bodies as often they want to or need to until they have completed their "spiritual education". The bodies are dispensable; the soul is eternal. But even if you only believe in one life, it still makes sense! Because if being human was our true self, then God would be human also! But God is not human so neither are we!

"From dust to dust, from ashes to ashes" is a cycle. Life is circular - the seasons, the crops and flowers, birth and death - it is all circular and eternal.