The nature of man is a very important doctrine. A lot of bad theology comes from ignorance as to this question of who we are and what makes a Christian.
DEFINITION: Trichotomy is the theory that a Christian person is made up of three inseparable aspects – body, soul and spirit. (Dichotomy says there is only body and soul; spirit is just another word for soul.)
The soul is commonly said to comprise our emotions, intellect, and will. (“Heart, head, hand.”) The Bible word “heart” often includes all three. The emotions were referred to as “reins, bowels,” etc., much like we may say, “I have a gut feeling.” The soul is “spiritual” by being invisible, religious (all men are), and moral (everybody has a conscience). Nevertheless, the soul by itself is non-spiritual, or “earthy”. The Bible speaks of animals as being “souls”. The soul is mortal. It is capable of evil as well as good. The soul of a person originates at conception. The best explanation as to its method of origin is traducian, that is, our personality is inherited from our forebears in the same way as our bodies. (The Bible word for the “natural” man is “psychical”.)
A person’s sprit is given him by God at the time of regeneration. It is something new (not a change in what he already has), which is why it is called a “second birth”. (Ezk 36:25,26) Such a person becomes a “new man”. It (he) is perfect, cannot sin. (I John 3:9) It is immortal; this is “eternal life”, which begins at the moment of conversion, and not when a person leaves this earth. It is the Holy Spirit (John 3:5), the Spirit of Christ (the God-man). This is “Christ in us” or being “in Christ”, an expression used 150 times in the New Testament. Needless to say, a born-again individual does not lose his identity by being “swallowed up in God”; it is like the “one flesh” which two persons become by means of marriage. (Ephesians 5:22-ff)
When a person becomes a Christian his old “man” is regarded as dead and buried (Romans 6:1-11, Col. 3:3, Gal 2:20). He is no longer depraved. He is no longer a “sinner” – saved, forgiven, or acquitted. He is now a saint. The two are mutually exclusive, just as a person cannot be regenerate and unregenerate at the same time.
Immediately upon regeneration the “new man”, the perfect spirit, begins to make the soul holy. This is what we usually regard as “sanctification”. (And it is Christ who is our sanctification in this sense as well as being the perfect person within us. I Cor. 1:30) The mind is renewed (Romans 12:2) so that we have the very “mind of Christ.” (I Cor. 2) The affections are centered on new objects (Col 3:2); his very will becomes “God’s will” (John 7:17; compare Romans 7:18 with Phil. 2:13). This process of perfection (Phil. 3:12) is completed at the moment of death.
The human body is the last to be “saved” (Rom. 8:23, II Cor 5:1-8, I Cor. 15:45-58), along with the physical creation (Rms 8:18-22, II Peter 3:10-13). But already here and now a Christian’s body shares in his present salvation (Ps. 103:3, Matt 8:18). This is because the soul and body affect each other for good as well as ill; God also effects miraculous healing of the mind and body whenever this serves His perfect purposes.
In addition to the texts quoted above, two important proofs of biblical evidence for trichotomy are I Thes. 5:23 and Hebrews 4:12. (The analogous doctrine of the Trinity – a word not found in scripture – has but 2 “proof texts”.) Old Testament texts are often quoted to “show” that soul and spirit are synonyms. However, the Spirit of Christ did not “exist” in the Old Testament (John 7:39) anymore than Jesus. Old Testament anthropology is as general as its doctrine of redemption and even of God, emphasizing His unity rather than tri-unity. (Harper Bible, p.1775).