For now, until I see a reason in the scriptures to think of it differently I will continue to use biblical language and use the word soul and spirit interchangeably as it relates to the unseen part of man and that which goes to be with Christ after the body dies. (for the believer of course) Here are some of the many scripture examples that form my biblical view.
Starting with the only one New Testament verse that can be used to suggest a three part view, and then the verse that states that a soul and spirit can be separated by the Word of God, these two verses do not by themselves give me any reason to modify my understanding of all the many verses that use soul and spirit interchangeably. At this time, (I am always open to change my mind based on sound interpretation of scriptures, (not human reasonings) ) and since the many responses in this thread so far, I am still thinking that the intention of 1 Thess 5:23 is to express... "your spirit and soul, as that immaterial part of you" (what you love, what you think, decisions you make about serving God) and your body being that physical part of you, how you stay sexually pure, etc. That you stay blameless and holy as you wait for the coming of the Lord. The same with Heb 4:12, that God sees the inside of our hearts and knows what we are really like and what our motives are. So there may be an intention here of saying that the Word of God can expose whether we are lying to ourselves. The poetic Joints and Marrow gives us reason to take the Spirit and Soul as poetic as well. As "if there is a way to separate soul and spirit it is the Word of God that can do that" I am still studying this from the Greek and I appear to be on the right track.
1 Thessalonians 5:23 (ESV): ‘Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ’.:
Heb 4:12 (NIV): ‘For the word of God is alive and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart’. Soul and spirit are clearly different words and they can be ‘divided’ according to this verse.
The Greek construction of 1 Thess 5:23 (transliterated):
hagiazw hymeis holoteleskai kai terew hymeis ho pneuma kai ho psyche kai ho swma terew holokleros
The literal translation is: ‘and may [the God of peace] sanctify you completely your (the) spirit and the soul and the body be preserved (completely) whole’.
Matt 10:28 (NIV): Do not be afraid of those who kill the body but cannot kill the soul. Rather, be afraid of the One who can destroy both soul and body in hell’.
Matt 22:37 (NIV): ‘Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’
Mark 8:36 (NLT): ‘And what do you benefit if you gain the whole world but lose your own soul?’
Mark 12:30 (NIV): ‘Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.’
1 Cor 7:34 (NIV), ‘An unmarried woman or virgin is concerned about the Lord’s affairs: Her aim is to be devoted to the Lord in both body and spirit. But a married woman is concerned about the affairs of this world—how she can please her husband’.
Rom 1:9 (NIV), ‘God, whom I serve in my spirit in preaching the gospel of his Son, is my witness how constantly I remember you’.
Genesis 35:18 (ESV): ‘And as her [Rachel’s] soul was departing (for she was dying), she called his name Ben-oni; but his father called him Benjamin’.
Matt 10:28 (ESV): ‘And do not fear those who kill the body but cannot kill the soul. Rather fear him who can destroy both soul and body in hell [Gehenna].
Rev 6:9 (NIV): ‘When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne’.
James 2:26 (ESV): ‘For as the body apart from the spirit is dead, so also faith apart from works is dead’.
Eccl 12:7 (ESV): ‘and the dust returns to the earth as it was, and the spirit returns to God who gave it’.
Luke 23:36 (ESV): ‘Then Jesus, calling out with a loud voice, said, “Father, into your hands I commit my spirit!” And having said this he breathed his last’.
Luke 8:54-55 (ESV): ‘But taking her by the hand he called, saying, “Child, arise.” And her spirit returned, and she got up at once. And he directed that something should be given to her to eat’.
Is this sending a contradictory message about the naming and function of the immaterial part of a human being? Some verses state the soul leaves the body at death while others take the view the spirit departs from the body. How do we resolve this difficulty?
the evidence points to the soul and spirit being evidence for the unseen part of human beings.