Many of us were reared in or introduced to a form of Christianity which taught what is referred to as "trichotomy," which means that man is composed of three parts: body, soul and spirit. Imagine my surprise when I discovered that this was never the orthodox position of the Church! I have heard similar expressions of shock from others who came from the same Pentecostal background that I came from. Traditionally, Christianity has always maintained that man is made of two parts: body and soul - and that soul and spirit are terms which are used interchangeably in Scripture.
A cynic might just ask, "So what?" After all isn't this obscurantist speculation? But upon further reflection, we should at least admit that ideas do have consequences.
Historically trichotomy has either contributed to or directly led to heresies. Apollinarius was the first prominent purveyor of trichotomy in Christian circles. He was quickly labeled a heretic. Because of its origins in Greek philosophy, trichotomy leads to the various forms of Gnosticism in the early Church. At its core Gnosticism almost always viewed matter as evil. Our spirits animate our bodies, they reasoned, thus they are bad too. But our souls, which are not corporeal, are pure and they are our tool to tap into true knowledge, secret knowledge, of God. Trichotomy, in every case in the early Church led to some form of Gnosticism. Among modern-day purveyors of this teaching (Pentecostals), it has led to Gnosticism as well, and frequently to Pelagianism. If nothing else, there is a historical warning for us.
Gnostic views have always led to either antinomianism or ascetic legalism. Created matter is either as an insufferable evil that must be put down at all costs. Thus it leads to asceticism. The monk vilifies his flesh and seeks to deprive it of all nourishment and comfort in order to promote the life of the soul. This works on the assumed premise that soul and flesh are at enmity with each other. At the other end of the spectrum, people have thought that since the immaterial realities of the soul are that which are truly important, then the body and spirit are irrelevant. Hence once one's soul is converted to God, it doesn't matter what the body does. This leads to all sorts of debauchery and vice under the guise of grace.
Scripture consistently present two natures in the universe corporeal and incorporeal - material and nonmaterial. Anyone who has done a word study on the terms soul and spirit can attest to the fact that these words are used interchangeably all throughout Scripture. Someone might rush in with Hebrews 4:12, which mentions dividing "soul and spirit." This proves nothing for the trichotomists case however. For the same verse mentions "joints and marrow," yet no one asserts from this wording that man is composed of four parts, two immaterial parts and two material parts. If the blade cuts one way, it cuts two ways - no pun intended.
When Charles Hodge deals with this in his Systematic Theology, he makes what I take to be a very powerful observation. Our own consciousness of ourselves testifies to our dichotomous nature. We are all consciously aware of our bodies as the material part of our being, but who, except for a schizophrenic, is consciously aware of TWO internal immaterial parts of his being?
Is this just obscurantist speculation? Think of the trichotomist impact on the person of Christ. Orthodox Christianity has always affirmed the Hypostatic Union of Christ. This means that Christ is fully God and fully man - two distinct natures, not intermingled - by a mystery of divine wisdom hypostatically united in One Person. Try to square this with trichotomy and you rush headlong into blasphemous heresy. You will end up trying to divide Christ in an Apollonarian way. This will make Christ a human body that had the Second Person of the Trinity, the Logos, in the place of a normal human soul. You must postulate two spiritual natures in Christ. You don't want to know where that will lead you.