19. Since there is but one God, why do you mention the Father, Son, and Holy Spirit, who are three?
Because in the one essence of God, we have to look on the Father as the beginning and origin, and the first cause of all things; then the Son, who is Eternal Wisdom; and the Holy Spirit who is His virtue and power shed abroad over all creatures, but still perpetually resident in Himself.
The Holy Spirit is mentioned twice in the Apostles’ Creed—once with respect to the birth of Christ, once in a belief statement about his Person in the Godhead – a statement that is parallel to the belief statements given toward the Father and the Son. His equality in Deity stands as a given in the Creed and the Catechism, and the study of his role in our redemption is part of the four principal parts of the Catechism’s exposition of the Creed for the strengthening of the church. By calling him “His virtue,” Calvin is not suggesting that the Spirit is an impersonal force; rather, it is the Spirit who comes as the Person of Truth on behalf of the Godhead so that God’s power and holiness might be placed (P)/personally in each member of God’s elect across the world. Yet, the Spirit is eternally God, being “perpetually resident in Himself” as God—the only Being with being within himself alone.
The Holy Spirit, for Calvin and the Fathers before him, is a member of the Triune God. From the period of the Creed to the Catechism there is a consistent witness that our faith is in one God eternally existing in three distinct Persons who are equal in power and glory. This follows very clear evidence in the text of Scripture that God is Triune—that the divinely-glorious Son has a relationship with the Person of the Father (Jn. 1:1-2; 17:4), and that the glory of that same Son will be magnified by the Person of the Spirit—the one who the Son promised to send from the Father. It is the Holy Spirit, the third Person of the Trinity, upon whom we rely to work the virtue of Christ into us, and by whom we accomplish God’s will in the strength of his might rather than the weakness of our human frailties.
The truth about the Spirit should encourage us to keep from being like both Mormons and Jehovah’s Witnesses in practice. Might we go to God in daily, earnest prayer, with contrition, requesting more of the Spirit. Might our churches, following the Creed, seek to be filled with people finding utter dependence on him. As Michael W. Smith sings, He should be to me like the air I breathe. He is perfect in power and love and purity.
Robert Letham, The Holy Trinity, in Scripture, History, Theology and Worship, (P&R, 2005).