Charles Ellicott Commentary 1 Kings 8:37-40

"If there be in the land famine, if there be pestilence, if there be blasting [or] mildew, locust [or] caterpillar; if their enemy besiege them in the land of their cities; whatsoever plague, whatsoever sickness there be; what prayer and supplication soever be made by any man, [or] by all thy people Israel, who shall know every man the plague of his own heart, and spread forth his hands toward this house: then hear thou in heaven thy dwelling-place, and forgive, and do, and render unto every man according to all his ways, whose heart thou knowest; (for thou, even thou only, knowest the hearts of all the children of men;) that they may fear thee all the days that they live in the land which thou gavest unto our fathers." — 1 Kings 8:37-40 (ASV)

If there be pestilence.—He then passes on to the various plagues threatened in the Law—famine, pestilence, blasting of the corn, mildew on the fruit, locust, and caterpillar (Deuteronomy 28:22–24; Deuteronomy 28:38–42). He also describes the distress of siege, so terribly depicted (Deuteronomy 28:52–57) and so often terribly fulfilled (not least in the last great siege of Jerusalem). To sum up all, he adds, whatsoever plague, whatsoever sickness there be.

Through any or all of these, he pictures each man as brought to know the plague of his own heart—that is, as startled into a consciousness of sin and recognition of it as the true "plague" (the cause of all outward plagues), and so drawn to prayer of penitence and godly fear.

You only, know the hearts ... of men. The emphasis placed on this knowledge of the heart (Psalms 139:2–4; Jeremiah 17:9–10) as the special attribute of Deity, though of course belonging to all vital religion, yet especially marks the leading thought of the Psalms and Proverbs. These books always realize God's presence not so much in the outer spheres of nature and history as in the soul of man itself.

This knowledge carries with it, as here, the conviction that under God’s general dealings of righteousness with humanity, there lies an individuality of judgment, making those dealings for each person exactly what their spiritual condition needs. The plague, for example, which cuts off one person unrepentant in their sins, may be to another a merciful deliverance out of the miseries of this sinful world.