As I briefly mention in my sermon, Paul’s words in his prayer in 1 Thessalonians 5:23 about being kept blameless in our “whole spirit and soul and body” are unlikely to be his attempt at explaining the constituent parts of humanity. Some, however, argue the opposite, and claim that Paul is presenting humanity as composed of three parts (trichotomy) over against two parts (dichotomy), specifically body and soul. But, Gordon Fee points out that Paul’s intention lies beyond this debate:
Most of the discussion on the three anthropological terms has centered either in determining whether or not Paul intended some kind of distinction between the first two terms [spirit and soul]—and if so, what?—or in the related question whether Paul was a dichotomist or trichotomist. That discussion, however, while not insignificant, has missed Paul’s point altogether. … Paul's concern is singular: that they be sanctified completely. In the context of this letter … the present emphasis lies with his inclusion of the body.
That is to say, much is made of the inclusion of spirit and soul, but Paul’s initial readers likely would have keyed in on his inclusion of the body as something that God cared to sanctify.
Now, while a discussion of trichotomy vs. dichotomy would be distracting in the sermon, this reflection is a worthwhile place to broaden the discussion to a general consideration of our composition as humans. First, we should take up Paul’s words in 1 Thessalonians 5 and their possible implications. Again, Gordon Fee is helpful:
What, then, shall we say of the first two terms? First, it is very likely, given the way Paul here expresses himself, that he might think of the human spirit and soul as distinct entities in some way. But how we might think of them in this way is not at all clear from the rest of his letters. Since he tends to use such terms both broadly and somewhat interchangeably, one is hard pressed to come to final conclusions. Moreover, the emphasis on entirety suggests that he could easily have included “mind” without for a moment deviating from his concern. That is, whatever distinctions he may have understood are quite secondary to the greater concern of completeness.
With this final statement, we hit at the most important point: humanity, though it can be described by its component parts, is more importantly described in its completeness or wholeness. We may temporarily draw out aspects of our human composition, e.g. mind, will, soul, etc., but these things must ultimately collapse back into unity. That is to say, there is no room for a radical separation of humanity into any number of parts, for we are a unity, that is, as Joel Beeke reminds us, only temporarily separated by the unnatural curse of death.
To briefly address the dichotomy vs. trichotomy question, though, the balance of the Scriptures’ witness leans in favor of dichotomy. As Fee points out, soul and spirit are often used interchangeably in the Scriptures. Furthermore, there is no clean way to parcel out our immaterial existence into more than one aspect. Moreover, there are only two realms of God’s creation, spiritual and material, and we would be hard pressed to divide out the spiritual into any further categories. Therefore, we are on more solid ground speaking about the psychosomatic (body-soul) unity of humanity as a way to give expression to how we reflect the unity-in-duality of God’s creation. In fact, Herman Bavinck points out that humanity is unique in its position as the convergence of the spiritual and material: “Creation culminates in humanity where the spiritual and material world are joined together.”
What all of this means for us is that we must keep in balance the importance of both our spiritual and material existence. For those that tend to emphasize our material existence, reflection on the body-soul unity of mankind can lift the eyes of their heart up to contemplate the wonder of an entire unseen existence that has great impact on our seen, or material, existence. For those that tend to emphasize our spiritual existence, reflection on the body-soul unity can ground spiritual contemplation in the intrinsic earthiness of our existence. Our goal is to be aware of and to balance our engagement with our spiritual and material existence so that we best reflect God’s intention when he created us.
 Gordon D. Fee, The First and Second Letters to the Thessalonians, NICNT (Grand Rapids: Eerdmans, 2009), 229–30.  Fee, Thessalonians, 230.  Herman Bavinck, Reformed Dogmatics, ed. John Bolt, trans. John Vriend, 4 vols. (Grand Rapids: Baker Academic, 2003), 2.511.