1 Kings 13:1
And, behold, there came a man of God out of Judah by the word of the LORD unto Beth-el: and Jeroboam stood by the altar to burn incense. And, behold, there came a man of God out of Judah by the word of the LORD unto Beth-el: and Jeroboam stood by the altar to burn incense.
The Lord sent a man of God out of Judah by the word of the Lord unto Bethel, and he arrived while Jeroboam stood by the altar to burn incense, interrupting the idolatrous offering in progress. The altar where the king ministered was unauthorized, set up apart from the true worship God had appointed. The coming was at the Lord's command, with a message for Jeroboam delivered directly against that altar.
The word spoken against the altar rebuked idolatrous sacrifices as an abomination to God. It set before the king and the ten tribes the evils of disobedience and of turning Israel away from worshiping God. It was meant to inspire awe and to turn the people to the Lord.
The message foretold that a descendant of David named Josiah would sacrifice the priests of the high places upon that very altar and burn human bones upon it. As a present sign, the altar would split apart and its ashes spill out, making the rebuke public and declaring God's displeasure with the idolatrous worship. That sign was fulfilled exactly as spoken.
The events that followed confirmed the prophecy spoken over the altar. When Jeroboam stretched out his hand to seize the messenger, his hand became helpless and palsied so he could not draw it back, and when he asked the man of God to pray for him his hand was healed. The later death of the disobedient messenger stood as a third evidence to Jeroboam that God's words were true, and together the rent altar, the palsied arm, and the prophet's death demonstrated God's swift displeasure.
These messages and signs should have led Jeroboam to repent, to abandon his evil ways and his plan to turn Israel away, and to serve the Lord. Instead he hardened his heart and chose his own way, and was warned by these judgments not to persist in wrongdoing. The signs were given to correct existing evils, but the people's hearts remained hardened. This reproof and entreaty to turn Israel from disobedience was one of repeated warnings later continued through Elijah and Elisha and through Amos and Hosea, yet Israel sank lower and lower in apostasy.