John 12:39

They could not believe,[3] that is, they would not, says St. Augustine, or it could not be, considering their wilful obstinate blindness. (Witham) — But where then is the sin, if they could not believe? They could not believe, because they would not. For as it is the glory of the will of God, that it cannot be averse to its own glory, so it is the fault of the will of man, that it cannot believe. (St. Augustine, tract. 53. in Joan.) They could not believe. Since the prophet has foretold it, and he cannot but say the truth, it is impossible that they should now believe. Not but they had it in their power to believe; and had they believed, the prophet would never have foretold the contrary. (St. Chrysostom, hom. lxvii. in Joan.)

Verses 37-43 Observe the method of conversion implied here. Sinners are brought to see the reality of Divine things, and to have some knowledge of them. To be converted, and truly turned from sin to Christ, as their Happiness and Portion. God will heal them, will justify and sanctify them; will pardon their sins, which are as bleeding wounds, and mortify their corruptions, which are as lurking diseases. See the power of the world in smothering convictions, from regard to the applause or censure of men. Love of the praise of men, as a by-end in that which is good, will make a man a hypocrite when religion is in fashion, and credit is to be got by it; and love of the praise of men, as a base principle in that which is evil, will make a man an apostate, when religion is in disgrace, and credit is to be lost for it.