Albert Barnes Commentary Zechariah 7:12

"Yea, they made their hearts as an adamant stone, lest they should hear the law, and the words which Jehovah of hosts had sent by his Spirit by the former prophets: therefore there came great wrath from Jehovah of hosts." — Zechariah 7:12 (ASV)

Harder than adamant - The stone, whatever it may be, was hard enough to cut ineffaceable characters. It was harder than flint. It would cut rocks; it could not be engraved itself, or receive the characters of God.

This is the last sin: obduracy, persevering impenitence, which resisted the Holy Spirit (Acts 7:51) and did despite to the Spirit of grace (Hebrews 10:29). Not through weakness, but deliberately, they hardened themselves, lest they should convert (Isaiah 6:10) and be healed. They feared to trust themselves to God’s word, lest He should convert them by it.

Lest they should hear the law and the words which the Lord God sent by His Spirit by the hand of the former prophets - The Holy Spirit was the chief agent—"by His Spirit"; the inspired prophets were His instruments—by the hand of the prophets. Nehemiah confesses the same to God: You did protest to them by Your Spirit by the hand of Your prophets (Nehemiah 9:30). Moses was one of the greatest prophets. The law then may be included, either as delivered by Moses, or as being continually enforced by all the prophets. Observe the gradations:

1. The words of God are not heard.

2. The restive shoulder is shown; people turn away when God, by the inner motions of His Spirit or by lesser chastisements, would bring them to the yoke of obedience. Osorius: “They would not bear the burden of the law, whereas they willingly bore that most heavy weight of their sins.”

3. Obduracy. Osorius: “Their adamantine heart could be softened neither by promises nor threats.” Therefore nothing remained but the great wrath, which they had treasured to themselves against the day of wrath. And so Zechariah returns to that with which his message and visions of future mercy began: the great wrath which fell upon their fathers (Zechariah 1:7).

Osorius: “‘I sought not,’ He says, ‘for your tears; I did not enjoin bitterness of sorrow. But what I commanded—had it been done—the calamity for which those tears were fitting would never have befallen you. What was it that I admonished you previously by the former prophets to recall you from sin? What I command you by Zechariah now. This I preach, admonish, testify, impress upon you.’”