Albert Barnes Commentary 1 Kings 13

Rather, “in the word of the Lord.” The meaning seems to be that the prophet was not merely commanded to come, but that he came in the strength and power of God’s word as a divinely inspired messenger. (Compare to 1 Kings 13:2, 1 Kings 13:5, and 1 Kings 13:32.)

By the altar means “on the altar”; that is, on the ledge or platform halfway up the altar, on which the officiating priest always stood to sacrifice. (Compare to the note on 1 Kings 12:32.)

A child shall be born ... Josiah by name - Divine predictions so seldom include such specific details that doubts are raised, even by orthodox theologians, regarding the actual mention of Josiah’s name by a prophet living in the time of Jeroboam. Only one other parallel instance occurs in all of Scripture: the mention of Cyrus by Isaiah. Of course, no one who believes in divine foreknowledge can doubt that God could, if He chose, cause events to be foretold in great detail by His prophets. However, it is a general principle of His providence that He does not do this. If this principle is to be broken, it will not be done arbitrarily.

In this case, it is not clear what great effect was produced by mentioning Josiah’s name so long before his birth, which raises the question of whether our present copies contain the true original text. The meaning is complete without the words “Josiah by name.” If these words were originally a marginal note, they could have easily crept into the text through a copyist's mistake. It is remarkable that in a later reference to this narrative in the book of Kings, there is no mention that the man of God had prophesied of Josiah “by name.”

He gave a sign - A sign of this kind—an immediate prophecy to prove the divine character of a remote prophecy—had scarcely been given before this. In later history, however, such signs are not infrequent (Compare to 2 Kings 19:29; Isaiah 7:14–16).

The ashes... shall be poured out - That is, “The half-burnt remains of the offerings shall be ignominiously spilled upon the ground.”

We do not need to imagine the altar was completely shattered. Instead, a crack or fissure likely appeared in its structure, extending from top to bottom. This break would have caused the embers and the pieces of the sacrificial offerings to spill out onto the ground.

I will give you a reward - It was customary to honor a prophet with a gift if he performed any service that was requested of him (see the marginal references).