1 Kings 13:3, commentary and pictures

At the dedication of Jeroboam son of Nebat's unconsecrated altar at Bethel, the man of God from Judah denounced the worship and gave an immediate sign the same day. That altar had been set up in bold defiance of God to turn Israel away from the temple in Jerusalem to new forms of worship where many in years to come would be seduced into idolatrous practices.

The sign was that the altar would be split apart and the ashes upon it poured out, proving that the word spoken against it was from the Lord and confirming the message, warning, and prophecy to Jeroboam. The sign came to pass as foretold and immediately, when the altar was rent and the ashes poured out, showing the Lord's warning being fulfilled. The sign interrupted the king's idolatrous and profane offering, restrained his power in his assault on the messenger, and was meant to arouse him from his idolatrous course.

The public splitting of the altar and pouring out of its ashes displayed God's displeasure with Israel's idolatrous worship and sacrifices, which were an abomination and could not make worship acceptable to Him. It was meant to make a deep impression on the people, awaken awe, lead them to repentance, and warn the king and people of the consequences of apostasy. The rent altar, together with the king's palsied arm and the death of the disobedient prophet who brought the message, formed three evidences confirming the truth of the prophecy and God's displeasure and warning Jeroboam to stop doing wrong, turn from evil, abandon his plans, and serve the Lord. He should have repented, but he hardened his heart and persisted in his chosen course.

The sign accompanied the announcement that a child named Josiah of the house of David would offer the priests of the high places upon that altar and burn men's bones upon it, a prophecy fulfilled literally three centuries later when Josiah stood at Bethel in his reformation.