I think that comprehension of God’s substance transcends not only human beings, but also every rational nature. Now by “rational nature” here, I mean one which belongs to creation. For the Father is known by the Son alone, and by the Holy Spirit. because: No one knows the Father except the Son (Matt 11.27), and: The Spirit searches everything, even the depths of God. For no one knows what belongs to a man except the spirit that is in him, and know one knows what belongs to God except the Spirit that is from God (1Cor. 2.10-11). What, then, will remain distinctive about the knowledge that the Only-Begotten or the Holy Spirit has, if indeed they themselves have comprehension of the very substance? Even though they do not attribute to the Only-Begotten the contemplation of the power and goodness and wisdom of God, they have nonetheless made the apprehension of God’s substance commensurate with themselves. In fact, the exact opposite is the case. It is to be expected that the very substance of God is incomprehensible to everyone except the Only-Begotten and the Holy Spirit. But we are led up from the activities of God and gain knowledge from the Maker through what he has made, and so come in this way to an understanding of his goodness and wisdom. For what can be known about God is that God has manifested (Rom. 1.19) to all human beings.
Since whatever the theologians seem to have recorded about the substance of God has been expressed in figurative language or even in allegories, the words transport us to other notions. Hence if someone should contentiously stand by the mere letter, taking it in its obvious interpretation without duly examining it, he has strayed into the myths of the Jews (Ti. 1.14) and silly old wives tales (1Tim 4.7), and will grow old in abject poverty, devoid of worthy concepts of God. For in addition to thinking that the substance of God is something material and thereby agreeing with the Greek atheists, he will also suppose that it is complex and composite. For example, the prophet describes God as like amber from his loins upward and composed of fire below (Ezek. 8.2). Whoever does not ascend, by means of the letter, to the loftier notions and somehow sticks to the corporeal descriptions of the passage learns from Ezekiel that this is what the substance of God is like. Then again, he will hear from Moses that God is fire (Dt. 4.24). In addition, the wise man Daniel will lead him to other suppositions. Hence when he reads the Scriptures he will find in them images which are not only false but also in conflict with one another.
Therefore, putting aside this idle curiosity about the substance since it is unattainable, we ought to obey the simple advice of the Apostle, who said: One must first believe that God exists and that he rewards those who seek him (Heb. 11.6). For it is not the investigation of what he is, but rather the confession that he is, which prepares salvation or us. Therefore, since it has been demonstrated that the substance of God is incomprehensible to human nature and completely ineffable, it remains that we must thoroughly examine unbeggotenness itself both what it is and how it is considered in the God of the universe.