Albert Barnes Commentary Psalms 7:9

"O let the wickedness of the wicked come to an end, but establish thou the righteous: For the righteous God trieth the minds and hearts." — Psalms 7:9 (ASV)

Oh let the wickedness of the wicked come to an end – This refers to the wickedness of all the wicked; not only wickedness in this particular case, but wickedness of all forms and in all lands. The prayer here is a natural one: when a person becomes impressed with a sense of the evil of sin in one form, they wish that the world may be delivered from it in all forms and altogether.

But establish the just – This refers to the righteous. This stands in contrast with the psalmist's desire concerning the wicked. The psalmist prays that the righteous may be confirmed in their integrity and that their plans may succeed. This prayer is as universal as the former and is, in fact, a prayer that the world may come under the dominion of the principles of truth and holiness.

For the righteous God trieth the hearts and reins – That is, the hearts and reins of all people. He understands the character of all people; he is intimately acquainted with all their thoughts, purposes, and feelings. To search or try the heart and the reins is an expression frequently used in the Bible to denote that God is intimately acquainted with all the thoughts and feelings of people; that is, that he thoroughly understands the character of all people. The word “heart” in the Scriptures is often used to denote the seat of the thoughts, and the word “reins” seems to be used to denote the most secret feelings, purposes, and devices of the soul—as if lodged deep in our nature or covered in the most hidden and concealed portions of a human being.

The word “reins,” for us, denotes the kidneys. In the Scriptures, the word seems to be used in a general sense to denote the inward parts, as the seat of the affections and passions.

The Hebrew word כליה (kilyâh)—means the same as the word “reins” for us: the kidneys (Exodus 29:13; Exodus 29:22; Job 16:13; Isaiah 34:6; Deuteronomy 32:14). For some reason, the Hebrews seem to have regarded the “reins” as the seat of the affections and passions, though perhaps only in the sense that they spoke of the “inward” parts in this manner, intending to denote the deepest purposes of the soul—as if utterly concealed from the eye.

These deep thoughts and feelings, so unknown to other people, are all known intimately to God. Thus, the character of every person is clearly understood by him, and he can judge every person aright. The phrase used here—trying the hearts and reins—is one that is often employed to describe the omniscience of God (Jeremiah 17:10; Jeremiah 20:12; Psalms 26:2; Psalms 139:13; Revelation 2:23). The particular idea here is that because God searches the hearts of all people and understands the secret purposes of the soul, he is able to judge aright and to determine correctly regarding their character, or to administer his government on the principles of exact justice.

Such is the ground of the prayer in this case: that God, who knew the character of all people, would confirm those who are truly righteous and would bring the wickedness of the ungodly to an end.