The authority of the biblical texts was a common frame of reference for all the parts involved in the Arian controversy. Still, Athanasius and the Cappadocians insisted that their rejection of Arian exegesis was the only consistent reading of the Bible. What were their reasons for maintaining this conclusion, and how are we to evaluate this position today? Are they, as modern Biblical scholarship has tended to argue, dependent on unbiblical notions of the essence of God that determine the way they read the biblical texts? Or are they right in insisting that all other ways of reading the texts are contradictory and inconsistent? Athanasius explained his Christology and its biblical foundation on several occasions through his life. The work that will be most closely investigated in this study, however, is his Defence of the Nicene Definition, which was written about 350, where the central topic is the use of the non-biblical word homoousios in the text produced by the Council of Nicaea. In defending this term, Athanasius emphasizes the difference between God and human, insisting that both impassibility and unknowability are necessary predicates of the divine, thus constituting the framework for any adequate interpretation of the relation between the eternal Father and his coeternal Son. What Athanasius here briefly touches, the question of divine unknowability, is the main topic of Gregory of Nyssa's Against Eunomius, where he refutes Eunomius's attempt at defining God as agenetos by insisting with the entire tradition of Christian apophaticism that God is essentially undefinable. The eternal Father's generation of his Son is therefore to kept free from associations from human fatherhood; still, it is a transmission of (unknowable) essence. The idea of homoousios therefore has to be accepted, not as a way of summarizing the doctrines of specific biblical passages, but as a way of guiding the reader to the gratuitous presence of the essentially unknowable One.