During the first thousand years following the birth of Christ numerous variant teachings arose attempting to replace Christianity with some religiosity of human invention and convenience.
All these variant teachings (heresies) were answered by the Church with the Seven Great Ecumenical Councils. At the first of these Councils in AD 325 at Nicaea the assembled Fathers proclaimed the Creed (from Credo, I believe) which summarizes the True Christian Doctrine.
This Nicene Creed as originally formulated is faithfully recited by the Orthodox at the Liturgy in all the churches in all places of Orthodox worship:
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 only-begotten Son of God; Begotten of his Father before all worlds, God of God, Light of Light, Very God of very God; Begotten, not made; Being of one substance with the Father; By whom all things were made: Who for us men and our Salvation came down from heaven, And was incarnate by the Holy Ghost of the Virgin Mary, And was made man: And was crucified also for us under Pontius Pilate; he suffered and was buried: And the third day he rose again according to the Scriptures: And ascended into heaven, And sitteth on the right hand of the Father: And he shall come again, with glory, to judge both the quick and the dead; Whose kingdom shall have no end.
And I believe in the Holy Ghost, The Lord, and Giver of Life, Who proceedeth from the Father; Who with the Father and Son together is worshipped and glorified; Who spake by the Prophets: And I believe one Holy, Catholic and Apostolic Church: I acknowledge one Baptism for the remission of sins: And I look for the Resurrection of the dead: And the life of the world to come. Amen.
Orthodox Christian worship is focused on God, not on our own enjoyment, fulfilment, or fellowship, and is formed by repentance, thanksgiving, and unending praise.
We come into the presence of God with awe, aware of our fallenness and His great mercy. We seek forgiveness and rejoice in the gift of salvation. Glory be to the Father, and to the Son, and to the Holy Ghost. As it was in the beginning, is now, and ever shall be. World without end. Amen.