The present study examines the theophanies of the Son and Word of God as a basic axis for the proof of the homoousion of the First Ecumenical Council. The verbal formulations of the earlier tradition are investigated and compared with the interpretative propositions of Athanasius the Great against the Arians. Theophanies are examined in the light of God the Word, who a) co-exists eternally with the Father, b) creates the world and c) reveals in the Old and the New Testament God the Father to the faithful.