dear Cold Steel...
you said:" Not only did they believe that all men have spirits, but that man existed prior to coming to Earth. (See Jeremiah 1:5)"
God knows the end from the beginning. He knows each of us...whether we will respond to His call on our life or not. He knows our destiny because He has seen it. He can provide opportunities for those who are called to minister effectively knowing that they will take ever opportunity to preach the word...those would be the very elect. Called to be sent from the very beginning because God knew them. The same "thing" for jeremiah as for all the other prophets that God had foreknowledge of...the apostles...all the way into our "time".
No human existed before we entered this earthly realm. Not even the flesh body of Jesus.(He always existed in union with the Father and the Holy Spirit as the Son of God) When (most) humans die they enter the "paradise" spoken of in luke (the rich man and lazarus) this is not the eternity where God resides.(the highest heaven) This is where the just and the unjust are resurrected from. (Some humans are translated into eternity, we have Jesus and elijah as known examples of this and the beloved apostle paul was also hoping for this "better resurrection", the resurrection of Christ he called it)
Because God resides outside of our time in eternity, it can be said that the end of our time has already been seen and "experienced" in that eternity by God and by those who have entered into it and it may be God's will that some be sent "back" into our "time" at God's will...elijah would again be an example of this, having made his appearance known twice already as himself, as JtB and scripture says he will come once more ...
anyway...we are told in the scriptures that the flesh comes first and then we are born of the Spirit through salvation which is in Jesus Christ alone.
love michelle