The Trinitarian doctrine is the foundation stone for the unity of the body of the Church. The Advocate, i.e. the Holy Spirit, deifies the human and transforms him into a member of the Church's body and a living proof of the unity with the other members of the Church's body. Therefore, the mission of the Church from the day of Pentecost until today, according to Eastern Orthodox Theology, is the preservation of the Trinitarian doctrine and its projection to humans. Also, the Church declares the Trinity's role in the "deification" and the unity of the Church's body. From the first to the twentieth century, this mission of the Church passed through four dimensions: the biblical, the patristic, the hymnological, and the synodical. In other words, I will present how the biblical narration about the Advocate's coming (Acts 2, 1-11) is the basis for the Cappadocean Fathers' Theology and the confrontation of the Eunomios' and Makedonios' teachings. Moreover, I will present how the Romanos the Melodist convert the Cappadocean Fathers' Trinitarian Theology into hymnology in the Kontakion of the Pentecost. Furthermore, I will also present how John the Damascene prescribes the Trinitarian Theology in his hymns for Pentecost, which were composed during the first period of the Iconoclasm dispute. The final stage of my paper will be the presentation of the Trinitarian Theology's inclusion into the Synodikon of Orthodoxy, the text that the Eastern Orthodox Church uses to emphatically recognize and declare its identity. The Easter Orthodox Church also projects the unifying and deifying character of the Trinity among its flock and towards the rest of the Christian world through the Synodikon.