Vainly then do they run about with the pretext that they have demanded Councils for the faith's sake; for divine Scripture is sufficient above all thingsHistory of Councils, 6
We do not speak of two gods, we do not think of the unity of the Son with the Father in terms of the similarity of their teaching, but in terms of being (ousia) and reality. So we speak...of one God who exists as one form of divinity, like the relationship of light to its ray.De synodis, 52.1
According to him [Athanasius] the unity of God as well as the distinctions in His being are best expressed in the term 'oneness of essence'. This clearly and unequivocally expresses the idea that the Son is of the same substance as the Father, but also implies that the two may differ in other respects, as, for instance, in personal subsistence.The History of Christian Doctrines (85)