|Mystical Icon of the 'Old Testament' Trinity, by Fr. Andrew Tregubov|
"And in the Holy Spirit, the Lord, the Giver of Life, who proceeds from the Father; who with the Father and the Son together is worshipped and glorified; who spoke by the prophets..." — from the Symbol of Faith
The Great Day of Pentecost is also designated as Trinity Sunday. On this day, we celebrate the final and full revelation of the Holy Trinity with the descent into the world of the Holy Spirit.
From all eternity the Holy Spirit "proceeds" from the Father; but He "proceeds" from the Son "in time" for it is the Risen Lord who sends the Paraclete (another name for the Holy Spirit) into the world on the Day of Pentecost.
Some of the Church's most profound and beautiful hymnography is found in the Feast of Pentecost - especially in the Vespers of Pentecost - when it comes to poetically revealing our understanding of God's trinitarian nature. As Orthodox Christians, we are monotheists, but we are trinitarian monotheists.
This following hymn - one of the apostikha of Vespers - stands on its own, with no need for commentary, though I will simply point out that this hymn "spells out" the trinitarian nature of the daily Trisagion Prayers that we offer up to God:
Come, let us worship the Tri-Personal
The Son in the Father with the Holy
The Father timelessly begets the co-
reigning and co-eternal Son.
The Holy Spirit was in the Father,
glorified equally with the Son,
One Power, One Substance, One God-
In worshipping Him, let us all say:
Holy God: who made all things
through the Son,
With the cooperation of the Spirit.
Holy Mighty: through whom we know
Through whom the Holy Spirit came
into the world!
Holy Immortal: the comforting Spirit,
Proceeding from the Father and resting
in the Son.
O Holy Trinity: glory to Thee!