1 Kings 13:5
The altar also was rent, and the ashes poured out from the altar, according to the sign which the man of God had given by the word of the LORD. The altar also was rent, and the ashes poured out from the altar, according to the sign which the man of God had given by the word of the LORD.
The altar at Bethel was split apart immediately and its ashes poured out exactly as the man of God had announced, and this fulfilled the sign given by the word of the Lord. The rent altar proved the message, prophecy, and warning concerning the altar true and confirmed that it had come by the word of the Lord. The sign confirmed the prophet's message against Jeroboam's false worship.
The rent altar was a public sign of God's displeasure with Israel's idolatrous worship, false worship, apostasy, and sacrifices. It rebuked that worship as an abomination to Him. The sign was meant to interrupt the proceedings, awaken awe and repentance, and warn the king and the people where their apostasy would lead. It warned what would happen to their supposedly sacred altars and warned Jeroboam against persisting in disobedience and continuing in wrongdoing.
The king's attempt to seize the man of God was restrained when the altar was destroyed. Even then some Israelites remained open to the Holy Spirit's influence. Together with the king's stricken arm and the disobedient prophet's death, the sign should have led Jeroboam to repent, abandon the plans that were turning Israel away from worshiping God, and turn from his evil ways to serve the Lord. But he hardened his heart and chose his own way.