This year (2025), when 1700 years have passed since the meeting of the First Ecumenical Council of Nicaea (325), the entire Christian world commemorates with great emotion this crucial event for the Christian Church by organizing congresses and symposia, or publishing documents, large-scale works, studies and articles in which its history, theology and importance are presented. In this con- text, our study is intended to be a contribution to the celebration of this council, a true pioneer for Christian theology. Since historical, theological literature, etc. published so far is immense, we will only highlight some aspects related to its history and theology, such as its convocation, the participating bishops, the way it was conducted, what issues were debated, the creed, the date of Easter celebration, the Meletian and Novatian schisms, as well as the canons approved there. KEYWORDS: First Ecumenical Council of Nicaea, councils, Emperor Constantine the Great, Arian heresy