This is the thing that ye shall do; A third part of you entering on the sabbath, of the priests and of the Levites, shall be porters of the doors;

King James Translators' Notes

doors: Heb. thresholds

Jamieson-Fausset-Brown Bible Commentary

4-9. This is the thing that ye shall do-The arrangements made for defense are here described. The people were divided into three bodies; one attended as guards to the king, while the other two were posted at all the doors and gates, and the captains and military officers who entered the temple unarmed to lull suspicion, were furnished with weapons out of the sacred armory, where David had deposited his trophies of victory and which was reopened on this occasion.

Matthew Henry's Concise Commentary

23:12-20 A warning from God was sent to Jehoram. The Spirit of prophecy might direct Elijah to prepare this writing in the foresight of Jehoram's crimes. He is plainly told that his sin should certainly ruin him. But no marvel that sinners are not frightened from sin, and to repentance, by the threatenings of misery in another world, when the certainty of misery in this world, the sinking of their estates, and the ruin of their health, will not restrain them from vicious courses. See Jehoram here stripped of all his comforts. Thus God plainly showed that the controversy was with him, and his house. He had slain all his brethren to strengthen himself; now, all his sons are slain but one. David's house must not be wholly destroyed, like those of Israel's kings, because a blessing was in it; that of the Messiah. Good men may be afflicted with diseases; but to them they are fatherly chastisements, and by the support of Divine consolations the soul may dwell at ease, even when the body lies in pain. To be sick and poor, sick and solitary, but especially to be sick and in sin, sick and under the curse of God, sick and without grace to bear it, is a most deplorable case. Wickedness and profaneness make men despicable, even in the eyes of those who have but little religion.