Therefore they could not believe, because that Esaias said again, Therefore they could not believe - Why? Because they did not believe the report of the prophets concerning Christ; therefore they credited not the miracles which he wrought as a proof that he was the person foretold by the prophets, and promised to their fathers. Having thus resisted the report of the prophets, and the evidence of Christ's own miracles, God gave them up to the darkness and hardness of their own hearts, so that they continued to reject every overture of Divine mercy; and God refused to heal their national wound, but, on the contrary, commissioned the Romans against them, so that their political existence was totally destroyed.
The prophecy of Isaiah was neither the cause nor the motive of their unbelief: it was a simple prediction, which imposed no necessity on them to resist the offers of mercy. They might have believed, notwithstanding the prediction, for such kinds of prophecies always include a tacit condition; they may believe, if they properly use the light and power which God has given them. Such prophecies also are of a general application - they will always suit somebody, for in every age persons will be found who resist the grace and Spirit of God like these disobedient Jews. However, it appears that this prediction belonged especially to these rejecters and crucifiers of Christ; and if the prophecy was infallible in its execution, with respect to them, it was not because of the prediction that they continued in unbelief, but because of their own voluntary obstinacy; and God foreseeing this, foretold it by the prophet. Should I say that, they could not believe, means, they would not believe, I should perhaps offend a generation of his children; and yet I am pretty certain the words should be so understood. However, that I may put myself under cover from all suspicion of perverting the meaning of a text which seems to some to be spoken in favor of that awful doctrine of unconditional reprobation, the very father of it shall interpret the text for me. Thus then saith St. Augustin: Quare autem non Poterant, si a me quaeratur, cito respondeo; Quia Nolebant: Malam quippe eorum Voluntatem praevidit Deus, et per prophetam praenunciavit. "If I be asked why they Could not believe? I immediately answer, Because They Would Not. And God, having foreseen their Bad Will, foretold it by the prophet." Aug. Tract. 53, in Joan.