What makes it clear that Adam & Eve could have chosen to never sin and been successful at it? In my opinion once God created them with the freedom to choose (which they obviously had)and also created them human, they were sure to sin, just like we do when we grow from children into adults, i think this is why God provided the tree of life (Jesus) right from the start, in the midst of the garden. I think they did eventually learn to eat (trust) in this tree and not to trust (eat) from the tree of knowledge (themselves, the law, thou shall not). The complete and perfect story is right here in the beginning if we just bother to dig it out through prayer, study and living. I think the doctrines of men, although well intentioned and having some good points, tend to lock us into themselves as we grow in our spiritual walk, if we allow them to. We should never allow what other people think be the final authority for what we think.