HOLY SPIRIT, trinitarian nature

The deity of the Holy Spirit ought to be clearly recognized in Scripture. Look at these facts: Christ is born; the Spirit is His forerunner. Christ is baptized; the Spirit bears witness. Christ is tempted; the Spirit leads Him up. Christ ascends; the Spirit takes His place. What great things are there in the character of God which are not found in the Spirit? What titles which belong to God are not also applied to Him? He is called the Spirit of God, the Spirit of Christ, the mind of Christ, the Spirit of the Lord, the Spirit of adoption, of truth, of liberty; the Spirit of wisdom, of understanding, of counsel, of might, of knowledge, of godliness, of the fear of God. This only begins to show how unlimited He is. 

Gregory of Nazianzus.