A mere three reasons for why we seek to understand the person and works of the Holy Spirit.
God is Triune
The systematic study of the Holy Spirit (pneumatology) is foundational to proper thinking about God and, consequently, proper thinking about oneself, one’s life in relation to God, and one’s relation to God’s creation. The question, ‘Why study the Holy Spirit?’ seems out of place. Why? Simply put, Christians worship a Triune God—Father, Son, and Spirit. To neglect or marginalize one of the persons of the Godhead (especially in one’s study) is to misunderstand who God is in himself, how it is that he acts both in the world and for the world, and how it is that human creatures should properly relate to him in worship.
The Spirit of God is the Gift of Godself to God’s people
For the Christian, the Holy Spirit is not merely God with us, as he was in the Incarnate Son, but God in each of us (now!). The Holy Spirit is the gift of Godself to the people of God. Upon the ascension of the Son, the Holy Spirit is sent to be the presence of God among and within the people of God. The entirety of the Christian life is the Spirit-indwelt life; God, in the third person of the Holy Spirit, makes his home in those who believe in him. Those who have been justified in Christ and participate in his life, death, resurrection, and ascension are gifted the Holy Spirit such that Christ’s work of redemption and reconciliation is applied and actualized in the life of every individual believing human creature. What this means is that as one, any one—pastor, child, elderly lady, charismatic, presbyterian, etc., journeys in the world, God himself, in the person of the Holy Spirit, is truly and intimately with her, transforming her from the inside out. In some real sense, the Holy Spirit is the person of the Godhead who is most directly available to redeemed humans. St Basil beautifully articulates this in his work On The Holy Spirit; he writes, “The Spirit is present to each one who is fit to receive him, as if he were present to him alone, and still he sends out grace that is complete and sufficient for all” (9,22). This truth of the Spirit’s intimate, immediate presence in us, to us, and for us should transform not merely our approach to ministry but the whole of our living.
The Spirit-indwelt Church is Witness to the Triune God
The church is the gathering of Spirit-indwelt people properly relating to God in worship and to one another in love. Because the Spirit of God unites the people of God by dwelling in each one that has been cleansed by the blood of Christ, the people of God are both one and many. The people of God, sanctified by the Spirit, are to be single-minded—a single creature. And yet, the church is and should be filled with a diversity of people. As C.S. Lewis notes in the preface to The Great Divorce, ‘Good, as it ripens, becomes continually more different not only from evil but from other good.’ As individual indwelt believers are conformed to Christ’s image by the Spirit of God, they, in a sense, become more and more unique—a sole reflection, a single glimpse of the goodness of God that can be made known only through that one. And this happens with each individual Spirit indwelt believer. Each believer becomes both more good (more conformed) and more distinct from other good. The picture here is that as the people of God are conformed to the image of God by the Spirit of God, the church is made the most explicit witness to the diversity and unity of the Triune God we serve.
So, why study pneumatology? God is Triune. God gifts himself and unites himself to the church in the person of the Holy Spirit. It is through this union between God and believers that God most clearly reveals his triune nature in his church—a single creature made up of a multiplicity of diverse persons.