Christians profess, in the words of the liturgy of St. John Chrysostom, “the Father and the Son and the Holy Spirit - the Trinity, Consubstantial and Indivisible.” At the basis of our faith lies a mystery, not fully comprehended by the human mind, and at the same time necessary for us to accept it on faith for our salvation - God is One and at the same time threefold in Persons. In His name - the Father, the Son and the Holy Spirit - we are all baptized. This brochure will popularly tell about the history of the revelation of the Triune God in the Old and New Testaments and the dogmatic foundations of the Orthodox Christian faith in the Holy Trinity.