The Son is thus the icon of the Father, and the Holy Spirit the icon of the Son. Whoever, therefore, has seen the Son, has seen the Father, and whoever has seen the Holy Spirit has seen the Son. As the Apostle says: The Lord is the Spirit; and again, The Spirit himself intercedes for us with sighs too deep for words...crying, Abba Father. He says rightly that the Lord is the Spirit when He cries "Abba! Father!" not that the Son is the Spirit - far from it! - but that the Son is seen and beheld in the Holy Spirit, and that never is the Son revealed without the Spirit nor the Spirit without the Son. Instead, it is in and through the Spirit that the Son Himself cries "Abba, Father!"
--St. Symeon the New Theologian, Third Ethical Discourse