Jehovah testing the kidneys and the heart. Jer 11:20.

In the same prophet,

Jehovah testing the righteous, seeing the kidneys and the heart. Jer 20:12.

In David,

You test the heart and the kidneys, a righteous God. Ps 7:9.

In the same author,

O Jehovah, explore my kidneys and my heart. Ps 26:2.

In the same author,

O Jehovah, You possess my kidneys. Ps 139:19.

In John,

I am He who has examined closely the kidneys and the heart. Rev 2:23.

Here 'the kidneys' means spiritual areas and 'the heart' celestial ones; that is, 'the kidneys' means areas of truth, while 'the heart' means areas of good. The reason for this is that the kidneys purify fluid whereas the heart purifies actual blood, and therefore 'testing, exploring, and examining closely the kidneys' means testing, exploring, and examining closely the amount and the essential nature of truth, that is, the amount and the essential nature of the faith present with a person. This meaning is also evident in Jeremiah,

Jehovah, You are near in their mouth but far away from their kidneys.

And in David,

Jehovah, behold, You desire truth in the kidneys. Ps 51:6.

The fact that in addition the ability to correct is attributed to the kidneys is also plain in David,

In the night seasons my kidneys correct me. Ps 16:7.