What prayer and supplication soever be made by any man, or by all thy people Israel, which shall know every man the plague of his own {o} heart, and spread forth his hands toward this house:

(o) For such are most suitable to receive God's mercies.

Wesley's Notes

8:38 The plague - His sin, which may be called the plague of his heart, in opposition to the other plagues here mentioned; so the sense is, who, by their afflictions are brought to a true and serious sense of their worse and inward plague of their sins, which are most fitly called the plague of the heart, because that is both the principal seat of sin, and the fountain from whence all actual sins flow.

Matthew Henry's Concise Commentary

8:22-53 In this excellent prayer, Solomon does as we should do in every prayer; he gives glory to God. Fresh experiences of the truth of God's promises call for larger praises. He sues for grace and favour from God. The experiences we have of God's performing his promises, should encourage us to depend upon them, and to plead them with him; and those who expect further mercies, must be thankful for former mercies. God's promises must be the guide of our desires, and the ground of our hopes and expectations in prayer. The sacrifices, the incense, and the whole service of the temple, were all typical of the Redeemer's offices, oblation, and intercession. The temple, therefore, was continually to be remembered. Under one word, forgive, Solomon expressed all that he could ask in behalf of his people. For, as all misery springs from sin, forgiveness of sin prepares the way for the removal of every evil, and the receiving of every good. Without it, no deliverance can prove a blessing. In addition to the teaching of the word of God, Solomon entreated the Lord himself to teach the people to profit by all, even by their chastisements. They shall know every man the plague of his own heart, what it is that pains him; and shall spread their hands in prayer toward this house; whether the trouble be of body or mind, they shall represent it before God. Inward burdens seem especially meant. Sin is the plague of our own hearts; our in-dwelling corruptions are our spiritual diseases: every true Israelite endeavours to know these, that he may mortify them, and watch against the risings of them. These drive him to his knees; lamenting these, he spreads forth his hands in prayer. After many particulars, Solomon concludes with the general request, that God would hearken to his praying people. No place, now, under the gospel, can add to the prayers made in or towards it. The substance is Christ; whatever we ask in his name, it shall be given us. In this manner the Israel of God is established and sanctified, the backslider is recovered and healed. In this manner the stranger is brought nigh, the mourner is comforted, the name of God is glorified. Sin is the cause of all our troubles; repentance and forgiveness lead to all human happiness.