For beginners prayer is like a joyous fire kindled in the heart; for the perfect, it is like a vigorous, sweet-scented light. Or again, prayer is the preaching of the Apostles, and action of faith, or rather, faith itself, "the substance of things hoped for" (Heb. 11:1)... the Gospel of God... a sign of purity, a token of holiness... baptism made manifest...f a pledge of the Holy Spirit... God's mercy... the seal of Christ, a ray of the noetic sun, the heart's dawn star, the confirmation of the Christian faith, the disclosure of reconciliation with God, God's grace, God's wisdom or, rather, the origin of true and absolute Wisdom; the revelation of God, the work of monks, the life of hesychasts, the source of stillness, an expression of the angelic state. Why say more? Prayer is God, who accomplishes everything in everyone, for there is a single action of Father, Son, and Holy Spirit, activating all things through Christ Jesus.
+St. Gregory the Sinaite, Commandments and Doctrines #113 in The Philokalia, volume 4