How can we know God? The only possible way is through His revelation to us. This is most appropriately communicated in the hymns for the Feast of Theophany (meaning "Manifestation of God"):
“When Thou, O Lord wast baptized in the Jordan,
the worship of the Trinity was made manifest.
For the voice of the Father bare 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!
One God in Three Persons
Orthodox Christians worship the Father, Son, and Holy Spirit—the Holy Trinity, the one God. Following the Holy Scriptures and the Church Fathers, the Church believes that the Trinity is three divine Persons who share one essence. It is paradoxical to believe thus, but it is how God has revealed Himself. All three Persons are consubstantial with each other, that is, they are of one essence and coeternal. There never was a time when any of the Persons of the Trinity ceased to exist. God is beyond and before time and yet acts within time, moving and speaking within history.
God the Father
The Father (Abba) is the fountainhead (source) of the Holy Trinity, and the Maker of heaven and earth, and of all things visible and invisible. God the Father created all things through the Son, in the Holy Spirit (Genesis 1 and 2; John 1:3; Job 33:4), and Orthodox Christians are called to worship Him (John 4:23). The Father loves us and sent His Son to give us everlasting life (John 3:16).
The scriptures reveal the one God is Three Persons: Father, Son, and Holy Spirit, eternally sharing the one divine nature. From the Father the Son is begotten before all ages and all time (Psalm 2:7; II Corinthians 11:31). It is also from the Father the Holy Spirit eternally proceeds (John 15:26). The Orthodox Church teaches that we come to know the Father through Jesus Christ, and in the Holy Spirit (Matthew 11:27).
In the Old Testament the intimate name of Father was not used to address God in prayer. Only in Christ and because of Christ can Christians have such boldness. Only Christians can properly say the Lord’s Prayer that was taught to them by the Son of God. Only those who have died and risen with Christ in baptism, and have received the power to become sons of God by the Holy Spirit in Chrismation are enabled to approach the Almighty God most high as their Father (John 1:12; Matthew 6:9; Romans 8:14; Galatians 4:4).
God the Son
The Son, Jesus Christ is the incarnate Second Person of the Holy Trinity, the only begotten Son of God, fully God and fully man, born in time of the Virgin Mary and begotten from before all time of God the Father. In the Bible, the Old Testament speaks of His incarnation (becoming man) and the Gospels and Epistles of the New Testament speak about His earthly life, death, and resurrection.
God The Holy Spirit
The Holy Spirit, or Holy Ghost is God, the third Person of the Holy Trinity, who, from eternity, “proceeds” from the Father (John 15:26). The Holy Spirit is co-equal with the Father and the Son. The word “Spirit” commonly translates the Greek New Testament word πνεῦμα (pnevma).
The Spirit dwells inside every true Christian, each one’s body being his temple (1 Cor. 3:16). He is depicted as a ‘Counselor’ or ‘Helper’ (paraclete in Greek), guiding them in the way of the truth. The ‘Fruit of the Spirit’ (i.e. the result of his work) is “love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control” (Galatians 5:22). The Spirit is also believed to give gifts or charismas (i.e. abilities) to Christians.
It is the Holy Spirit whom Jesus mentioned as the promised “Comforter” (i.e. “strengthener”, “fortifier”) in John 14:26. After his resurrection, Christ told his disciples that they would be “baptized with the Holy Spirit”, and would receive power from on high (Acts 1:4-8); this promise was fulfilled in the events recorded in the second chapter of Acts. At Pentecost, Jesus’ disciples were gathered in Jerusalem when a mighty wind was heard and tongues of fire appeared over their heads. A multilingual crowd heard the disciples speaking, and each of them heard them speaking in his or her native language by the grace of the Holy Spirit.
The Nicene Creed
This belief is confessed daily by Orthodox Christians who seek to enter into God’s revelation of Himself for their salvation. Not only in personal prayer, but also in liturgical worship do we confess this together:
I believe in one God, the Father Almighty, Maker of Heaven and Earth and of all things visible and invisible.
And in one Lord Jesus Christ, the Son of God, the only-begotten, begotten of the Father before all ages. Light of light; true God of true God; begotten, not made; of one essence with the Father, by Whom all things were made; Who for us men and for our salvation came down from Heaven, and was incarnate of the Holy Spirit and the Virgin Mary, and became man. And He was crucified for us under Pontius Pilate, and suffered, and was buried. And the third day He arose again, according to the Scriptures, and ascended into Heaven, and sits at the right hand of the Father; and He shall come again with glory to judge the living and the dead; Whose Kingdom shall have no end.
And in the Holy Spirit, the Lord, the Giver of Life, Who proceeds from the Father; Who with the Father and the Son together is worshipped and glorified; Who spoke by the prophets.
In one Holy, Catholic, and Apostolic Church. I acknowledge one baptism for the remission of sins. I look for the resurrection of the dead and the life of the world to come. Amen.