Bless the LORD, O my soul, and all that is within me, bless his holy name! - Psalm 103:1 (ESV)
How to live for God with your soul? Between our heart, soul, mind, and strength, This is perhaps the deepest of the four aspects with which we ought to love God, and therefore live for him.
Why is this? We have said that our soul is our life itself. The soul is our substance, the material of which we ourselves are made. We may lose a finger, an arm, or even our bodies, and yet retain our existence. This is because we are souls at our most fundamental level. Our soul is the base of our existence. It is the substance beyond which there is nothing that defines us, that distinguishes us as beings distinct from others. Our soul is our being itself. So far as we can tell, the soul is indivisible and irreducible.
Our Souls, Our Selves
Our soul must be understood to be spirit, for we do not see souls as we normally conceive of seeing. In speaking of souls, therefore, we are venturing into the metaphysical, the realm of thought which does not assume time and space but seeks to examine them. Yet because we are fundamentally limited by space and time, it is something like a contradiction to conceive of thinking about them from the outside. We can't conceive of being "outside" of space without using the terminology of space.
All this means that when we speak of living for God with our soul, we are speaking of the most basic denominator of our existence. When David writes in Psalm 103, "Bless the LORD, o my soul, and all that is within me, bless his holy name", he writes with the intention of summoning his entire being to worship.
At first glance, this seems a strange thing to do. Why would a person, a being most fundamentally a soul, call on his own substance to do what his substance was already doing? We remember that the term "soul" has a wider meaning than "inner being". David could be referring to his appetite, his mind, or his desires. However, David's second line, "and all that is within me", seems to indicate David is indeed summoning up every fiber of his being to bless Yahweh.
The idea of integrity may help us understand what David is doing. Integrity refers to a lack of division, to wholeness, that a thing is the same through and through (like a pearl). David seems to be concerned with not only presenting his lips to God in praise of him but his whole self. David wants to hold nothing back. David wants every fiber of his being to be devoted to worship.
No Holding Back
In our living for God, it is all too easy to hold back, to reserve some part of ourselves for ourselves or for something else. When we consider living for God with our soul, we are talking about full, unreserved, total devotion of our being to God.
How do we do this with our soul? If we consider our hearts as the seat of our faculties - our thinking, desiring, and choosing - we can consider the soul the actual canvas upon (or within?) which those faculties find their subsistence. This is to say that when we use the term "heart" to refer to thinking, desiring, and choosing, we are using figurative language because we know that the physical heart in our chests pumps blood and that there are more organ systems involved in these faculties than the cardiovascular system. We can at least name the endocrinological and nervous systems as having a part to play. "Heart" does not refer to the organ but is figurative language that helps us put difficult notions to easy functional use. It is hard to be precise in language when speaking in terms of function is so much more immediately and frequently demanded than speaking in terms of form. We default to figurative language because it is easy, but a failure to understand the literal nature of things will inevitably result in errors big and small.
"Soul" is not figurative. It is literal, and nonetheless so for being something other than strictly physical as we typically conceive of the term. So when we speak of living for God with our soul in addition to with our heart, we are attempting to go beyond the involvement of the functions of our soul to the form of our soul itself.
In other words, I do not want to live for God only with regard to those things which I conceive of as being able to submit or devote to him in some way. I want to live for God with regard to all that I am in myself. I myself, at the very core of my being, to the deepest level, want every part of who and what I am to live for God through Christ. If I restrict my living for God only to those faculties which my soul possesses which I call "heart", I can conceive of withholding myself as a being from God. But that is not the goal. The goal is to live for God with all that I am in myself and not only all that I can offer God from myself. I want to offer not only the contents but also the container.
Perhaps the above discussion can give new meaning to the phrase, "Show me what you're made of". When we speak of living for God with our soul, we are talking about peeling back any and all superficial layers down to who we really are. Rather than putting up a front, a facade, we attempt to make our core selves the front which we display to God.
Perhaps the thing people most react to in this regard is the requirement of brutal honesty and openness before God. Attempting to cover up and hide from God formed part of the immediate aftermath of the disobedience in the Garden of Eden. In coming to Christ, we are united once again with God and promised a place in paradise with Him. Through Christ, we can live for God with our soul because there is no longer any reason to cover up and hide.