The meaning is this. Isaiah introduces words spoken as if by God: if I become man and with My own voice teach you what is profitable, even then you will not obey Me, just as you did not obey the prophets.
Even when you see miracles beyond reason, you will receive no profit from seeing. This is what the expression "you will not see" means.
He did not say, "I will harden their hearts and blind their eyes." Rather, He says that if you hear, you will not hear, and if you see, you will not see, so that I will not heal you when you turn to Me.
If they had heard and seen in the proper way, they would certainly have profited. Therefore the saying does not indicate a necessary punishment, nor the decree of a Judge condemning the Jews.
It serves as a foreannouncement of what is profitable, for He knew what they would be and declared it about them. Yet the word is not directed against all, but only against the unbelievers, for many of them believed.
The Seventy translated this passage in this way. It is likely, however, that the Evangelist, following the Hebrew text, which differs from the Septuagint, said: "Therefore they could not believe," because He had blinded them.
As to the direct meaning of the saying, he did not say that God blinded them. Naturally, this is because another did it, so that the Jews would not turn and receive healing.
Even if we accept that God blinded them, it must be understood this way: He permitted them to suffer blindness from the devil, since they were not good in their moral condition.
In this way He also "gave over" people of a similar moral condition, the Gentiles, "to a debased mind" (Rom. 1:28) and to passions.
While they were such, it was not just for them to know the depth of the mystery and the hidden truths, since they had not kept even the commandments under the law.
Therefore, if they received neither the law nor the Gospel ordinances, having closed the eye of their soul, then for this reason they do not receive the instruction that could enlighten them.
He utters many accusations against the Jews. Many signs did not shame them, but they were somehow unbending in their hardness of heart, though nothing had been omitted that could lead them to correction.
Even if the Jews, being ashamed, had believed, it was still impossible for the Holy Spirit to lie (Acts 5:3), who spoke through the prophet Isaiah concerning their unbelief.
They did not believe the prophets either, and Christ, the power and arm of the Father, was not made known to the Jews. That no one believed is said hyperbolically, because out of many only a few were saved.
Wishing to expose their boundless corruption, the Evangelist added the words "before them," showing that they did not believe even what they saw.
"Lord! ... to whom has the arm of the Lord been revealed?" The arm (βραχίων) of God the Father is the Son. This thought appears many times in the saint, often in almost the same words.
Compare: "The right hand (δεξιά) and arm of God the Father is the Son" (Commentarius in Isaiam prophetam 1:5, PG 70.256A). Also: "The arm of God and the Father, that is, the Son" (ibid. V.1, PG 70.1161B).
Also: "That the right hand and arm of the Father is the Son" (Thesaurus 35, PG 75.636C).