Barnes New Testament Notes

Verse 39. They could not believe. See Mr 6:5. "He could there do no mighty works," &c. The words can and could are often used in the Bible to denote the existence of such obstacles as to make a result certain, or as affirming that while one thing exists another thing cannot follow. Thus, Joh 5:44: "How can ye believe which receive honour one of another." That is, while this propensity to seek for honour exists, it will effectually prevent your believing. Thus (Ge 37:4) it is said of the brethren of Joseph that they "could not speak peaceably unto him." That is, while their hatred continued so strong, the other result would follow. See also Mt 12:34; Ro 8:7; Joh 6:60; Am 3:3.

In this case it means that there was some obstacle or difficulty that made it certain that while it existed they would not believe. What that was is stated in the next verse; and while that blindness of mind and that hardness of heart existed, it was impossible that they should believe, for the two things were incompatible. But this determines nothing about their power of removing that blindness, or of yielding their heart to the gospel. It simply affirms that while one exists the other cannot follow. Chrysostom and Augustine understand this of a moral inability, and not of any natural want of power. "They could not, because they would not" (Chrysostom in loco). So on Jer 13:23, "Can the Ethiopian change his skin," &c., he says, "he does not say it is impossible for a wicked man to do well, but, BECAUSE they will not, therefore they cannot." Augustine says on this place: "If I be asked why they could not believe, I answer without hesitation, because they would not: because God foresaw their evil will, and he announced it beforehand by the prophet."

Said again, Isa 6:9,10.