Jehovah trieth the reins and the heart (Jer. 11:20). In the same:

Jehovah that trieth the righteous, and seeth the reins and the heart (Jer. 20:12). In David:

The just God trieth the hearts and reins (Ps. 7:9). Again:

O Jehovah explore my reins and my heart (Ps. 26:2). Again:

Jehovah Thou hast possessed my reins (Ps. 139:13). In Revelation:

I am He that searcheth the reins and the heart (Rev. 2:23). In these passages spiritual things are signified by the "reins" [or "kidneys"] and celestial things by the "heart;" that is, the things which are of truth are signified by the "reins," and those which are of good by the "heart." The reason of this is that the kidneys purify the serum, and the heart purifies the blood itself; hence by "trying, exploring, and searching the reins," is signified to try, explore, and search out the quantity and quality of truth, or the quantity and quality of faith, in man. That this is the signification is plain also in Jeremiah:

Jehovah Thou art near in their mouth, but far from their reins (Jer. 12:2);

and in David:

Jehovah, behold Thou desirest truth in the reins (Ps. 51:6). That "chastening" is attributed to the kidneys is clear also in David:

My reins chasten me in the night seasons (Ps. 16:7).