The most central mysteries of our faith—the Trinity, the Incarnation, and the redemption that Christ reveals in his Paschal Sacrifice—were attested in the Scriptures and are proclaimed and celebrated in the Eucharist. They were formulated with precision over time by the Church’s Magisterium (the teaching office of the Church) to keep the communities that read the Scriptures and celebrated the Eucharist in the same communion of right understanding and right worship (orthodoxy) about these things, a communion that was to hold across the whole world and through the centuries.
The great Doctor of the Church St. Augustine of Hippo spent over 30 years working on his treatise De Trinitate [about the Holy Trinity], endeavoring to conceive an intelligible explanation for the mystery of the Trinity.
He (St. Augustine) was walking by the seashore one day contemplating and trying to understand the mystery of the Holy Trinity when he saw a small boy running back and forth from the water to a spot on the seashore. The boy was using a sea shell to carry the water from the ocean and place it into a small hole in the sand. The Bishop of Hippo approached him and asked, “My boy, what are doing?” “I am trying to bring all the sea into this hole,” the boy replied with a sweet smile. “But that is impossible, my dear child, the hole cannot contain all that water” said Augustine.
The boy paused in his work, stood up, looked into the eyes of the Saint, and replied, “It is no more impossible than what you are trying to do – comprehend the immensity of the mystery of the Holy Trinity with your small intelligence.”
The Saint was absorbed by such a keen response from that child and turned his eyes from him for a short while. When he glanced down to ask him something else, the boy had vanished. Some say that it was an Angel sent by God to teach Augustine a lesson on pride in learning. Others affirm it was the Christ Child Himself who appeared to the Saint to remind him of the limits of human understanding before the great mysteries of our Faith. Through this story, the sea shell has become a symbol of St. Augustine and the study of theology.
In describing the unity of essence of the Father, Son, and Holy Spirit, St. Augustine uses a very simple approach that is easy to comprehend. First the Father, Son, and Holy Spirit are one and are equal to each other. They are three distinct persons, but one God. This is also seen in scripture at the Baptism of Christ. Christ is present; there is the Holy Spirit in form of the dove, and the Father’s voice from Heaven. St. Augustine describes it as the Son being begotten from the Father, but the Father was not born of the Virgin Mary, and Christ did not descend on the faithful at Pentecost. The three members of the Trinity work as one and are in perfect unity.