Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed.

Wesley's Notes

6:10 Fat - Stupid and senseless. This making of their hearts fat, is here ascribed to the prophet, as it is ascribed to God in the repetition of this prophecy, John 12:40, because God inflicted this judgment upon them by the ministry of the prophet, partly by way of prediction, foretelling that this would be the effect of his preaching; and partly by withdrawing the light and help of his Spirit. Heavy - Make them dull of hearing. Lest - That they may not be able, as before they were not willing to see. Convert - Turn to God.

Jamieson-Fausset-Brown Bible Commentary

10. Make . fat-(Ps 119:17). "Render them the more hardened by thy warnings" [Maurer]. This effect is the fruit, not of the truth in itself, but of the corrupt state of their hearts, to which God here judicially gives them over (Isa 63:17). Gesenius takes the imperatives as futures. "Proclaim truths, the result of which proclamation will be their becoming the more hardened" (Ro 1:28; Eph 4:18); but this does not so well as the former set forth God as designedly giving up sinners to judicial hardening (Ro 11:8; 2Th 2:11). In the first member of the sentence, the order is, the heart, ears, eyes; in the latter, the reverse order, the eyes, ears, heart. It is from the heart that corruption flows into the ears and eyes (Mr 7:21, 22); but through the eyes and ears healing reaches the heart (Ro 10:17), [Bengel]. (Jer 5:21; Eze 12:2; Zec 7:11; Ac 7:57; 2Ti 4:4). In Mt 13:15, the words are quoted in the indicative, "is waxed gross" (so the Septuagint), not the imperative, "make fat"; God's word as to the future is as certain as if it were already fulfilled. To see with one's eyes will not convince a will that is opposed to the truth (compare Joh 11:45, 46; 12:10, 11). "One must love divine things in order to understand them" [Pascal].

be healed-of their spiritual malady, sin (Isa 1:6; Ps 103:3; Jer 17:14).

Matthew Henry's Concise Commentary

6:9-13 God sends Isaiah to foretell the ruin of his people. Many hear the sound of God's word, but do not feel the power of it. God sometimes, in righteous judgment, gives men up to blindness of mind, because they will not receive the truth in the love of it. But no humble inquirer after Christ, need to fear this awful doom, which is a spiritual judgment on those who will still hold fast their sins. Let every one pray for the enlightening of the Holy Spirit, that he may perceive how precious are the Divine mercies, by which alone we are secured against this dreadful danger. Yet the Lord would preserve a remnant, like the tenth, holy to him. And blessed be God, he still preserves his church; however professors or visible churches may be lopped off as unfruitful, the holy seed will shoot forth, from whom all the numerous branches of righteousness shall arise.