Make the heart fat
Whether the prophet's commission hardens hearts as judgment or describes a result, and how the quotations differ.
Isaiah 6:9-10
9He said, “Go, and tell this people, ‘You hear indeed, but don’t understand. You see indeed, but don’t perceive.’
10Make the heart of this people fat. Make their ears heavy, and shut their eyes; lest they see with their eyes, hear with their ears, understand with their heart, and turn again, and be healed.”
What each tradition says
Rabbinic / Midrashic
The Jewish Encyclopedia reads Isaiah as offering himself as the Lord's spokesman with no personal consideration holding him back, unlike Jeremiah. Though assured no exhortation would affect the callous consciences of his hearers, he still went in and out among his people as if hope existed.
Emil G. Hirsch, Thomas Kelly Cheyne, Isidore Singer, Isaac Broydé, Isaiah (Jewish Encyclopedia, 1901-1906) — Article 'Isaiah' (on the call narrative, ch. vi)
Patristic
Chrysostom says the “because” and “spake” refer not to the cause of their unbelief but to the event. They did not disbelieve because Isaiah spoke; rather, because they were not about to believe, he spoke.
John Chrysostom, Homilies on the Gospel of St. John (NPNF I/14) — Homily 68, on John xii. 34-50
Catholic
Aquinas distinguishes two elements in spiritual blindness and hardness of heart. As a movement of the human mind cleaving to evil and turning from the divine light, God is not its cause, just as he is not the cause of sin. But as the withdrawal of grace, leaving the mind unenlightened and the heart unsoftened, God is its cause.
Thomas Aquinas, Summa Theologica (Fathers of the English Dominican Province trans.) — I-II, q. 79, a. 3 ('Whether God is the cause of spiritual blindness and hardness of heart')
Orthodox
No sourced position found. Checked CCEL NPNF II/9 (John of Damascus, Hilary of Poitiers), NPNF II/4 (Athanasius), the Chrysostom volumes (ANF/NPNF), and pravoslavieto.com (Philaret's Longer Catechism): no public-domain English Orthodox comment on Isa. 6:9-10; Theophylact's Isaiah commentary survives only in modern copyrighted translation.
Reformed / Protestant
Calvin holds that God warns the prophet in advance that his teaching will produce no good effect. Yet God commands him to teach anyway, so he should not regret or stop, but obey and leave the results to God.
John Calvin, John Calvin's Bible Commentary (Calvin Translation Society; 'Commentary on Isaiah') — Commentary on Isaiah 6:9
Evangelical
Barnes says the expressions denote hardness of heart and blindness of mind. The people would hear the prophet’s words but not understand them, because they were so stubbornly set on sin that they would neither believe nor regard him.
Albert Barnes, Albert Barnes' Bible Commentary — On Isaiah 6:9