This is the most important question of the day but for most people it doesn't even arise. Either they think there is no such thing as a soul, a spiritual element to their being, or else they believe their soul is who and what they feel themselves to be here and now. Something no different to their everyday personality. I would say even most religious people don't know what the soul is, especially those who don't believe it exists before birth in this world, so let me attempt a definition. The question is important because if we know what we really are then we would know what we should be doing and where we should be going. If we don't know what we are how can we determine the correct way to think or act or where to direct our aspiration?
The soul is the core of our being. It is that aspect of the self that is joined to the eternal and which takes us beyond the mundane and the material and imparts the sense of the sacred. It is the source of our individuality and our freedom. Without the soul these essential elements of what it is to be human would not exist. They do not derive from matter which is why a materialistic approach to life has no explanation for them and must, if it is to be logically consistent which often it is not, deny they exist.
The soul is a spiritual being which is to say its natural focus is the spiritual realm which is the plane of consciousness as it is in itself. In a sense, it is pure subjectivity but it needs the experience of objectivity, otherwise known as duality, to bring it out of itself and enable it to grow which means to become first aware of and then able to engage with the wider reality of God's creation, and indeed God himself. The soul is naturally one with God but passively so. To become aware of God and to know God in a deeper way, to consciously share in his greater being, it must externalise itself which it does in the world where there is I and That, subject and object. The challenges it faces in such an environment give it the opportunity to grow.
However, this is a process with risk attached. When the soul leaves its spiritually secure state it is neither good nor evil. The tendency to one or the other condition is brought out by its reaction to the dualistic world. It can consciously align itself with the greater reality of God or it can reject this reality and become fixated on itself. This goes deeper than simple belief. I call it the orientation of the heart. It is a matter of feeling. Not emotion which is based on personal reaction to stimuli but innate response to spiritual reality, a kind of answering resonance of the self to God. This should make clear that true religion is not following a set of rules or teachings but the positive opening of the heart to the higher spiritual power, higher because there are many forces in the universe but we are talking about the Creator not anything in creation.
The Masters told me that the greater part of you remains with us. This implies that only an aspect of the soul takes earthly incarnation. The soul is more than the earthly personality which is its representative in three dimensional existence but the two are intimately linked forming a whole. What one does impacts the other even if only a fraction of the totality of what the soul is manifests in our ordinary existence.
It is the soul that evolves which means unfolds its spiritual potential, but it does so by means of the earthly personality and what it experiences and expresses in this world. There is a symbiotic relationship between the two and although ultimately only the soul remains, it is coloured by the personality which, as it were, eventually dissolves into it as sugar might do into tea. The wise do not identify themselves with their personality but do not reject it either. It is part of the totality of their being but not the central core. That remains the soul.
Of course, there are spiritual states even beyond the soul though to differentiate between the two I would call these higher states divine. We are called to conjoin our soul with these higher states in divine union but even when that takes place our individuality remains what it is. Expanded, deepened, divinised, but still the soul.