We Christians are monotheists, that is, we believe in one God. This is something we share with Jews and Moslems and with some others. However, we also believe that God, while being one in essence, is also three persons: Father, Son and Holy Spirit.
God is one but three. This teaching of the Trinity is where we differ from Jews and Moslems.
Although the doctrine of the Trinity was only clearly revealed in the New Testament, when we as Christians read the Old Testament we see hints of the Trinity.
For example in Genesis1:26, God says “Let us make man in our image”. Notice the plural pronouns. In Genesis 18 three angels appear to Abraham but Abraham addressed them with the singular “you” (This distinction is lost in modern English. Older Bible translations use “thou” here.) In Isaiah 6, when the prophet sees a vision of God in the temple in Jerusalem the angels sing “Holy, Holy, Holy”. Notice the threefold repetition. There are other examples.
However, if the doctrine of the Trinity is foreshadowed in the Old Testament, we see it more clearly in the New Testament, beginning with the Baptism of Jesus Christ, which we celebrate on January 6th. This event is described in all four Gospels with slight variations. Basically, Jesus Christ goes down in the waters of the Jordan river, the Holy Spirit appears as a dove and the voice of God the Father is heard saying “This is my beloved Son”. This is the first clear mention of the Father, the Son and the Holy Spirit.
As we read through the Gospels and the Epistles we see many references to the Trinity but it actually took several centuries for the church to understand how God can be three and one. This doctrine is expressed in the Creed we sing at every Liturgy where we see that God is one in essence or substance but three “persons”.
The doctrine of the Trinity shows us that the Divine Persons have existed in love for one another for all eternity, before the creation of humanity. This shows us that just as God exists in loving relationship, we too, as made in the image and likeness of God, are made for relationships with the persons of the Holy Trinity, the Mother of God and the saints and other human beings. As the poet John Donne says “No man is an island.”
Troparion for Theophany (Tone 1)
Во Иордане крещающуся Тебе, Господи, Троическое явися поклонение: Родителев бо глас свидетельствоваше Тебе, возлюбленнаго Тя Сына именуя, и Дух в виде голубине, извествоваше словесе утверждение. Явлейся, Христе Боже и мир просвещей, слава Тебе.
When Thou, O Lord, wast baptized in the Jordan the worship of the Trinity was made manifest. For the voice of the Father bore witness to Thee, and called Thee His beloved Son. And the Spirit, in the form of a dove, confirmed the truthfulness of His word. O Christ, our God, who hast revealed Thyself and hast enlightened the world, glory to Thee.