“… ; and all the churches shall know that I am he which searcheth the reins and hearts: and I will give unto every one of you according to your works.” (Re 2:23 AV)

Many people will not understand what is being said here because we do not usually use that old English word “reins” any more.  It is actually referring to the kidneys.  Of course this is a figure of speech just as the reference to the heart is.  Newer translations actually interpret the Greek work “nephros” as mind.  Although it in deed is speaking of the mind, much can be lost if we ignore the figurative language of the original.  When it says that God searches the reins and the heart it is not simply referring to those physical organs in our body, but figuratively speaking of those parts of our spiritual being that are most important.  Everything that we hold dear in our heart has first passed through our mind.  By using the word kidney to refer to our mind we become mindful of the function of our mind that is like unto the kidney.  Just as the kidney filters out those things that would poison the body, so a health mind will filter out those things that would poison our spiritual being.  God is letting us know that he examines both our spiritual heart and our mind.  The great physician would have our mind filtering out those thoughts and ideas that could potentially poison us, leading to a unhealthy heart.  Heart disease is even more deadly in the spiritual.

“I the LORD search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings.” (Jer 17:10 AV)