In the Western Church, the Sunday following Pentecost is Trinity Sunday. As point of comparison the Byzantine Church today is the Sunday of All Saints.
There are plenty of things to say about the Holy Trinity but I think this hymn attributed to St. Jacob of Sarug found today in the Syriac Maronite at the Third Hour of Sunday, says all of what we believe.
Glory be to the Father Who chose the prophets and they foreshadowed His Only-Begotten Son. Worship be to the Son Whose Gospel the apostles proclaimed in the world. Thanks be to the Spirit Who strengthened the martyrs and they died in His hope, and may mercy be upon us sinners through their prayers.
Today, I am also reminded of the life and writings of Saint Elizabeth of the Trinity and Blessed Itala Mela (Maria of the Trinity). The first a Discalced Carmelite and the second a Benedictine Oblate. Both devoted their lives to make this piece of dogma, this beautiful mystery, known and lived.
Blessed Itala said, “The will of Christ, which I feel in the depths of my soul, is to drag me, to immerse myself with Himself in the abysses of the Holy Trinity … It is useless to look for other ways: this is what He has chosen for my sanctification.”