Now may the God of peace Himself sanctify you completely; and may your whole spirit, soul, and body be preserved blameless at the coming of our Lord Jesus Christ. (1 Thes 5:23)
This scripture shows that we have three parts: spirit, soul, and body. Many scriptures also make reference to the mind and the heart. These, I believe, are found in the soul. I want to talk mainly about the spirit, but to do so, let me first say what I think the soul and the body consist of in this context.
Our body is the physical substance of our existence. It is made up of atoms and molecules that form increasingly complex structures that provide for our lives on this physical earth. Some say this is all we are and develop complex worldviews to explain how the body is the sum total of us. For example, Naturalism based on evolution teaches that the body is all there is and that our thoughts and feelings are a product of chemical reactions. Existentialism is a worldview that tried to resolve the problems that Naturalism is confronted with by saying that there is an objective reality - physical reality - and a subjective reality, the world of the mind and heart. I'm sure my generalizations will offend the naturalists and existentialists, but its just a generalization - not a complete summary of their worldviews. Regardless, I disagree with them both.
My worldview, Biblical Theism, recognizes something beyond the physical. The soul is part of this worldview's explanation of that which lies beyond the physical. The soul is the conglomeration of our personhood - our intellect, thoughts, desires, emotions. Within the soul lies the mind where we reason, calculate, contemplate, and conclude. Also within the soul lies the heart wherein we love and hate and experience all the range of human emotion. The mind thinks, the heart feels - but both make up who we are, the person, the personality - these are the soul.
The spirit is very different. The spirit is that part of us that relates to another. It is the part of us the gets connected to another when we have relationships. As people, I believe we were made primarily for relationship. The highest calling of man is to relate to another being. Every part of us is built to accomodate that calling - to have relationships with other people. If you look around you in the world, most human endeavors culminate in the search for companionship, love, and acceptance - the desire to be a part of something bigger than the self. It is the spirit that draws us in that direction.
When we have a relationship with another person, that relationship is a spiritual thing. Our spirits connect us to each other. You might think that a spiritual person is a "holy" person. Actually, the spirit isn't moral or immoral - it's amoral. It just connects - to whatever we decide to connect it to. The bible says, "He who is slow to anger is better than the mighty, And he who rules his spirit than he who takes a city." (Prov 16:32) It is up to us to rule our spirits well - to connect them approriately to that which does us good and not harm.
God has a Spirit - we call His Spirit the Holy Spirit. We were built to connect with the Holy Spirit. It is the connection above all connections. You may have heard people say, "We all have a God hole." Which is to say, there is an emptiness within each of us that can only be filled by God. Our spirit longs to connect with it's Creator. We need this connection - as a matter of fact, the bible says that we are sustained by this connection and broken without it: "The spirit of a man will sustain him in sickness, but who can bear a broken spirit?" (Prov 18:14)
We crave connection beyond all cravings. We are like electical plugs in search of an outlet. This plug has many options however. While it was made, first and foremost, to connect with the Creator - it can connect nearly anywhere. There are many imposters. We can substitute human relationships in the place of this God relationship - and so many of us do. We can substitute a relationship with substances, such as drugs, to fill this vacuum. Some people "connect" with food, others with alcohol, some with sex. There are lots of ways to temporarily satisfy the spirit with an imposter - but all imposters will eventually leave us unsatisfied and in need of an ever greater "dose" of the imposter.
The Bible refers to these imposters as "idols." God says, "connect with Me - let Me make you whole." We often say, "but I can't see you." We fall for our idols because they are physical. Humanity craves that which is physical. We want to see it, touch it, feel it, experience it. So with our spirit we often connect to the idol rather than the Real Thing. Connecting with the Real Thing requires faith, a subject for another time.
A spirit that is properly connected to its creator, can then properly connect with other people. When I am properly "relating" to God, all my other relationships go well - especially with my wife. My marriage depends on my being properly connected to God. If I am not responding to the leadership of the Holy Spirit, I tend to walk all over my wife, and I start to see her as a means to fill my wants rather than a person with whom I have a relationship.
I've said in another post that we have relationships that are of mutual benefit to each party. For a relationship to be beneficial, it must have terms. We formalize those terms when we get married via the wedding vows. A term might be: "forsaking all others". In other relationships, the terms are implied. In business relationships, terms are written in contracts. Regardless of the relationship, it is the terms that ensure that we receive the benefit of the relationship.
When terms are broken, the relationship is damaged - there is a severing that takes place in the spirit. When a married man looks at another woman, his spouse feels a spiritual assault. She senses that the terms of her relationship are at risk - the spiritual connection most important to her is about to be weakened. A woman gives her spirit, her soul, and her body to her husband when they marry. He enjoys those as benefits. She receives many benefits as well, but when he looks at another woman, she fears that she may have given that which is most precious away for nothing.
Sin is how the bible labels our breaking of the terms of the spiritual connection we have with God. When we sin, we deprive God of the benefit of our devotion, love, praise, etc. Instead we are focused on that which brings us pleasure (often to our detriment). We are connecting to an idol (relating to it) rather than to God. This is a spiritual assault against our connection with God. It hurts the relationship we have with Him. It is a spiritual disconnection. As a result, He withdraws from us. We lose the benefits for which we originally joined ourselves to Him. The Bible says that "the kingdom of God is not eating and drinking, but righteousness and peace and joy in the Holy Spirit." (Romans 14:17) When we disconnect from God, we lose the benefits of the Kingdom of God - righteousness, peace, and joy.
Since our connection with others depends on our connection with God, sin is also an assault on all of our human relationships as well. God asks us not to sin not only for His sake, but also for our sake. First, He doesn't want to lose the benefits of our relationship with Him, but He also doesn't want us to damage our relationships with others. Our human relationships are very important. They are the vehicle by which all good things enter our lives, and also the vehicle by which good things enter the lives of others. Above all, our relationships are the vehicle through which God is introduced to non-believers.
The spirit is that part of us the connects, that has relationships. It is probably the most important part of us, considering that we were made for relationships. But being made in the image of God however, means that we are body, soul, and spirit just like He is (Jesus Christ, Father, Holy Spirit) and God values every part of us.