God’s Word, although it might be on the lips of the wicked ones, does not reach down to the innermost aspects of their personality. Thus, regarding the wicked ones, Jeremiah 12:2 says: “They keep going ahead; they have also produced fruit. You are near in their mouth, but far away from their kidneys.” This parallels the scripture at Isaiah 29:13, quoted by Jesus at Matthew 15:7, 8, which says the same thing about the heart of the wicked, thus showing that at times “heart” and “kidneys” are used in a corresponding way.

At times kidneys are mentioned in close connection with the heart, which is used figuratively for the total inner person.​—Jer 11:20; 17:10; 20:12; see HEART.

Our Creator knows the makeup of man in the most thorough and intimate way. Therefore, Jehovah is said to test out “heart and kidneys,” even as his Son also searches “the kidneys and hearts.” (Ps 7:9; Re 2:23) Just as one refines silver, Jehovah can “refine” the kidneys and heart of a person so he may become right before God, being made more sensitive to Jehovah’s ways.​—Ps 26:2; 66:10.