Observe that when Plato says, that “after having found out the Creator and Father of the universe, it is impossible to make Him known to all men,” he does not speak of Him as unspeakable, and as incapable of being expressed in words. On the contrary, he implies that He may be spoken of, and that there are a few to whom He may be made known. But Celsus, as if forgetting the language which he had just quoted from Plato, immediately gives God the name of “the unspeakable.” He says: “since the wise men have found out this way, in order to be able to give us some idea of the First of Beings, who is unspeakable.”
For ourselves, we hold that not God alone is unspeakable, but other things also which are inferior to Him. Such are the things which Paul labours to express when he says, “I heard unspeakable words, which it is not lawful for a man to utter,” where the word “heard” is used in the sense of “understood;” as in the passage, “He who has ears to hear, let him hear.” We also hold that it is a hard matter to see the Creator and Father of the universe; but it is possible to see Him in the way thus referred to, “Blessed are the pure in heart, for they shall see God;” and not only so, but also in the sense of the words of Him “who is the image of the invisible God;” “He who has seen Me has seen the Father who sent Me.” No sensible person could suppose that these last words were spoken in reference to His bodily presence, which was open to the view of all; otherwise all those who said, “Crucify him, crucify him,” and Pilate, who had power over the humanity of Jesus, were among those who saw God the Father, which is absurd.
Moreover, that these words, “He that has seen Me, has seen the Father who sent Me,” are not to be taken in their grosser sense, is plain from the answer which He gave to Philip, “Have I been so long time with you, and yet do you not know Me, Philip?” after Philip had asked, “Show us the Father, and it suffices us.” He, then, who perceives how these words, “The Word was made flesh,” are to be understood of the only-begotten Son of God, the first-born of all creation, will also understand how, in seeing the image of the invisible God, we see “the Creator and Father of the universe.”
Source: Against Celsus (New Advent)