God has been revealing himself to humanity from earliest times. In the first reading, he appeared to Moses in the form of a cloud, a symbol of divine presence repeated through the Old and New Testaments. God gave him the Law in the form of the Ten Commandments, which bound the Jewish people together and distinguished them as worshippers of the one God. They imagined him as a king, enthroned on high as in Isaiah’s vision (Is. 6:1-4). There are hints of the three persons, for example, in the appearance of three men to Abraham (Gen. 18:1-3) and in the Book of Daniel, the image of the “one like a human being” coming to the Ancient One and being given dominion (Daniel 7:13-15). However the concept of the triune nature of God had yet to develop.
In the New Testament Jesus, God Incarnate, began to reveal more about the Divine Persons. He told parables such as the Prodigal Son (Luke 15:11-32) to show more clearly what God was like. In the Discources of St John (John 4-16) Jesus spoke about his Father and the Spirit. He told Philip that to see him was to see the Father (John 14:8-11). He described the work of the Holy Spirit, who would guide the infant church (John 16:12-14).
The early church reflected on the knowledge of God that had been given, but it was not until the Councils of Nicaea (AD 325) and Constantinople (AD 381) that the doctrine of the Trinity was finally defined. The three persons of the Trinity are engaged in a relationship of pure love. At our baptism we enter into their life and remind ourselves of our relationship with them each time we make the sign of the cross.
I bind unto myself today,
the strong name of the Trinity
by invocation of the same,
the Three in One and One in Three— St Patrick’s Breastplate