We believe in one God. This God is trinitarian. That is to say, God includes three persons: the Father, the Son, and the Holy Spirit. But if He is three persons, how can He be one God? This is the greatest mystery that the human mind can absorb. It does not perplex only us. We are not the only ones who cannot understand by rational means the great mystery of the Holy Trinity. Great wise men and the great Fathers of the Church had the same problem. One Father of the Church, St. Augustine, studied the mystery of the Holy Trinity and although he was at his wits' end, he still could not comprehend it. One day he was walking on the sandy beach by the ocean. There churned in his mind the mystery of the Holy Trinity. He was talking to himself: "One God, but three Persons. Three Persons--not three Gods but one God. What does it mean? How can it be explained? How can my mind take it in?" And so he was torturing his mind and beating his brains out, when he saw a little boy on the beach. He approached him to see what he was doing.
The child had dug a small hole in the sand. With his little hands he was carrying water from the ocean and was dumping it in the little hole. St. Augustine asked, "What are you doing, my child?" The child replied, "I want to put all of the water of the ocean into this hole." Once more St. Augustine asked, "But is it possible for all of the water of this great ocean to be contained in this little hole?" And the child asked him in return, "If the water of the ocean cannot be contained in this little hole, then how can the Infinite Trinitarian God be contained in your mind?" And the child disappeared. He was actually a little angel.
St. Augustine learned his lesson. He reverently thanked God, Who taught him in such a miraculous way that the mystery of the Holy Trinity cannot be comprehended with human reasoning. It is a matter of faith rather than of human reasoning. Whoever believes in God lives the mystery of the Holy Trinity and does not require rational proof.
We ourselves cannot prove rationally the mystery of the Holy Trinity. It would not then be a mystery. However, we shall mention a few verses from the Old and the New Testament that speak about the Holy Trinity and do not leave any doubt that God is One, but trinitarian. He has three Persons.
In the Old Testament, the emphasis falls mostly on God as one. Moses shouted aloud to the Israelites, "Hear, O Israel! The Lord our God is One" (Deuteronomy, 6:4). In spite of this, even in the Old Testament there are indications and references to the Holy Trinity. Not clearly and plainly, but in a cloudy way and veiled. There are many verses in which God is presented as being comprised of more than one Person. At the creation of man we read, "And God said, Let Us make man in Our own image and likeness" (Genesis, 1:26). When men had committed many sins and when they were building the Tower of Babel not for the glory of God but for their own glory, in order to bring them to their senses, God decided to "confuse their tongues." And God said, "Let Us go down and there We will confuse their tongues." There are other references in the Old Testament where God speaks in the plural. But why so? He is not using the "royal plural" form of the verb. Simply put, this plural form refers to the Three Persons of the Holy Trinity, the Father, the Son, and the Holy Spirit. More concretely, the Prophet Isaiah remarks that the angels, the Seraphim, fly about the throne of God and offer praises to Him, saying "Holy, holy, holy, Lord of the Sabbath, the entire earth is filled with His glory." Why "holy" three times? As the Fathers of the Church explain, it refers to the three Persons of the Holy Trinity. "Holy" is the Father. "Holy" is the Son. "Holy" is the Holy Spirit. In addition we have the hospitality of Abraham. There the three Persons of the Holy Trinity appeared as three angels.
The New Testament teaches about the mystery of the Holy Trinity more explicitly. When Christ was baptized in the Jordan River we have the appearance of the Holy Trinity. Christ was being baptized. The Holy Spirit descended in the form of a dove. The Father exclaimed from Heaven, "This is my beloved Son in Whom I am well pleased." For this reason the holy day for this event is called Theophany (the appearance of God). And after His Resurrection Christ said to His Disciples, "Go and teach all nations, baptizing them in the name of the Father and the Son and the Holy Spirit." St. Paul greets the Corinthians, "The Grace of our Lord Jesus Christ and the love of God and the communion of the Holy Spirit be with you all." With all of these verses and many more, the New Testament does not leave any doubt that God is trinitarian. We cannot ask for an explanation of the mystery. It would not be a mystery if it had an explanation. We accept the truth of God and we understand it and we live it only through our faith.
Thrice Holy One, Trinitarian God, Father, Son, and Holy Spirit, we thank You that You have revealed Yourself to us. Together with the Seraphim, we glorify You, saying Holy is the Father, Holy is the Son, Holy is the Holy Spirit. We sinners humbly beseech You. Grant us peace and serenity. Lead the whole world to know You as You are, Trinitarian and Most Holy. Let no one be lost. Bring all to true faith. Unite us with Yourself. Keep us in Your love and in Your Kingdom.