This becomes particularly clear in the last sentence of Timaeus: the accumulation of positive superlatives for this world (μέγιστος χαὶ ἄριστος χάλλιστός τε χαὶ τελεώτατος . . . μονολεγής) is only possible because this sensible world is an image of the intelligible world, in which we find unchangeable being. Similarly in Athanasius” view, the ontological foundation of Christ’ divinity gives stability and reliability to God’s revelation. It should be noted that the Arians confessed Christ as the Son of God, but they were opposed to the use of the terms οὐσία and ὁμούοσιος. Against them Athanasius fervently maintains that Christ’s sonship must refer to His οὐσία. The reason for this. it seems to us, lies in the fact that Athanasius believes that if God is not by essence what He does or what He is to us, then He might cease doing what He does or cease being what He is to. So Athanasius says that if God were not eternally Father, but being the Father were added to His essence, then God would be changeable (i.e. He could cease being the Father). Elsewhere he says that, if God started being good and were not good in essence, then He could cease being good. Similarly he says of Christ that if He were not God’s wisdom in essence, but learned God’s wisdom, He could unlearn it as well. So to Athanasius the reliability of God’s revelation requires that God in essence not different from what He does. If God reveals himself in the Son, then God must be in essence the eternal Father of the eternal Son, otherwise man cannot trust God’s revelation, since God could then cease doing what He does. We have already see that the fact God is unchangeable is very important to Athanasius. The reason is that God is faithful only if He is unchangeable. We get the impression that the ontological foundation of God’s revelation in Christ satisfied Athanasius’ desire for security, a general human desire.
The other reason why Athanasius stresses the ontological godhead if the Son is, as already been observed, that he believed that only God can redeem man, and that Christ, as Redeemer, must therefore be God.