The Procession of the Spirit — John 15:26

John 15:26 speaks of "the Spirit of truth, who proceeds from the Father." On this verse hangs the doctrine of procession and, eventually, the long filioque controversy between East and West.

Augustine — On the Trinity, a formula the West added to Council of Nicaea — Nicene Creed and the East rejected. Gregory of Nazianzus — Theological Orations.

The Spirit's witnessing role connects to regeneration in John 3:6-8 and to Pentecost's promise in Acts 2:17. The same Spirit who proceeds is poured out on the church.