The Holy Spirit subsists in the indivisible Trinity as one of the three hypostases (persons). As such, He is fully and exhaustively God, one in being eternally with the Father and the Son, one in power and glory. Whatever God does, the Spirit does, since in all God’s works all three persons work together inseparably, whether in creation, providence, or salvation. Therefore, when we speak of the Spirit at work, we must always remember that the Father and the Son are also involved.
Nevertheless, the Holy Spirit is not the Father, and He is not the Son either, for the three are eternally distinct. There is but one God, so the Spirit is identical in being or essence with the Father and the Son, but in terms of personhood, He is irreducibly distinct. Thus, there are actions attributed (or appropriated) peculiarly to the Spirit—only He was sent at Pentecost—but even here He was sent by the Father through and in the Son.
In terms of the eternal relations of the three, the Spirit proceeds from the Father (John 15:26). In this sense, the Spirit is from the Father, a relation that entails no element of subordination, inferiority, or temporal precedence but rather points to a relational and hypostatic (personal) order. This is something beyond our capacity to understand, as it occurs in the mystery of the internal life of God. However, by faith we seek to understand.
In line with this eternal procession, the Father sends the Spirit, through and in the Son, in relation to all His works in creation, including our redemption (Acts 1:8; Gal. 4:4–6). This is known as a mission (sending).
As noted, the Son is also actively engaged with the Father in the sending of the Spirit. Jesus refers to the Father’s sending the Spirit at Pentecost in response to His request or in His name (John 14:16, 26). He also says that He Himself will send the Spirit (16:7), and later He breathes on the disciples and says, “Receive the Holy Spirit” (20:22).
This has given rise to endless debate as to how the Son is involved eternally in the relations between the Father and the Spirit. There is no explicit biblical statement on whether the Spirit proceeds from the Son as well as from the Father. Originally, the Nicene Creed merely recorded that the Spirit “proceeds from the Father.” The Latin church later added the phrase filioque (“and the Son”) to the creed, to the vehement and continued ecclesiastical and theological opposition of the Eastern churches. The West understood that the Father had committed all things to the Son, including the spiration (or procession) of the Spirit, while the East maintains that this jeopardizes the Father as the source of the personal subsistence of both Son and Spirit and, moreover, blurs the distinction between the Son and the Spirit. However, both the Eastern churches (the Eastern Orthodox) and the Western churches (Roman Catholicism and Protestantism) agree that since the Trinity is indivisible, all three are integrally involved.