This article offers a comprehensive interpretation of the Council of Nicaea, in light of Eusebius of Caesarea’s role in the so-called Arian crisis. Given the historical-theological orientation of the study, it begins with hermeneutical caveats regarding the sources. It then examines the outbreak of the crisis, Eusebius’s theology before the controversy, the Ossius embassy, the Council of Antioch, and the Council of Nicaea itself. The article argues, first, that the key theological issue at stake was the strict eternity of the Son, which Eusebius of Caesarea denied; second, that Eusebius—not Arius—was the principal adversary of Alexander of Alexandria; third, that the Nicene theological discussions primarily revolved around Eusebius’s faith; and, fourth, that the homoousios implied the strict eternity of the Son.