St. Symeon the New Theologian (949-1022) was a 10th century mystic monk. His many direct experiences of God (theoría) convinced him that all Christians must have an actual baptism of the Holy Spirit in addition to the ritualized water baptism and chrismation of the church.
His mystical experiences also taught him that God is always received in the form of divine light. Symeon wrote about that light and its power to transform:
“It shines on us without evening, without change, without alteration, without form. It speaks, works, lives, gives life, and changes into light those whom it illuminates. We bear witness that “God is light,” and those to whom it has been granted to see Him have all beheld Him as light. Those who have seen Him have received Him as light, because the light of His glory goes before Him, and it is impossible for Him to appear without light. Those who have not seen His light have not seen Him, for He is the light, and those who have not received the light have not yet received grace. Those who have received grace have received the light of God and have received God, even as Christ Himself, who is the Light, has said, “I will live in them and move among them.” (2 Cor. 6:16) ~ Discourses, No. XXVIII