Christians believe in God the Trinity — Father, Son and Holy Spirit. The Trinity is not three gods, but one God in three Hypostases, in three personal beings. What mathematics and logic consider an absurdity constitutes the cornerstone of our faith, namely that 1=3 and 3=1. Christians participate in the trinitarian Godhead not through logic but through repentance, that is, through a complete change and renewal of the mind, heart and feelings (the Greek word for ‘repentance’ — metanoia — literally means ‘change of mind’). To touch upon the mystery of the Holy Trinity is impossible unless the mind changes from a rational way of thinking and becomes illumined by divine grace.
The doctrine of the Trinity is not an invention of theologians, not a teaching which gradually developed within the Church, but divinely revealed truth. At the Baptism of Jesus Christ, God reveals Himself in all clarity to the world as Unity in three Persons: ‘Now when all the people were baptized, and when Jesus also had been baptized and was praying, the heaven was opened, and the Holy Spirit descended upon him in bodily form, as a dove, and a voice came from heaven: Thou art my beloved Son; with thee I am well pleased’ (Luke 3:21-22). The voice of the Father is heard from the heavens, the Son stands in the waters of the Jordan, and the Spirit descends upon Him.
Jesus Christ repeatedly speaks of His unity with the Father, and of His being sent into the world by the Father. He also promises to send His disciples the Spirit, the Comforter, Who proceeds from the Father (John 14:16-17; 15:26). Sending His disciples out into the world to preach, He says to them: ‘Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit’ (Matt 28:19), which becomes the baptismal formula of the early Christian Church. The apostles themselves refer to the three Persons: ‘There are three witnesses in the heavens; Father, Son and Holy Spirit, and these three agree’ (1 John 5:7).
At the Incarnation of the Word God revealed Himself to the world as One in three Persons. The Jews, who had preserved their sacred faith in the one God, would have been unable to grasp the idea of a Divine Trinity as this would unmistakably have been taken to mean polytheism. At a time when polytheistic religion ruled the world, the mystery of the Trinity was hidden from human gaze. It was hidden as if it were in the very depths, in the very heart of the dogma of the divine unity.