A kidney is an internal organ of the body. In the Bible, kidneys had both a physical and spiritual meaning.
In ancient Israel, God commanded that the kidneys of sacrificial animals, along with their fat, be burned as offerings on the altar (Exodus 29:13; Leviticus 3:4–15). This was part of God's instructions about which parts of animals could and could not be eaten. The Israelites were not allowed to eat blood, and the kidneys represented this restriction.
The Bible also uses the kidneys as a symbol for a person's deepest thoughts and feelings. Hebrew writers often mentioned kidneys alongside the heart and soul to describe someone's inner self or true character (Psalm 73:21). They believed the kidneys were the place where people made moral choices and thought deeply about life (Psalm 16:7; Jeremiah 12:2). The Hebrew word for kidneys is often translated as "heart."
In the Old Testament, God knows everyone's deepest thoughts (for example, Psalms 7:9; 26:2; Jeremiah 20:12). So, Jesus was identified in the book of Revelation as "the One who searches minds and hearts" (Revelation 2:23). This indirectly but clearly identifies Jesus with God. This is the only reference to the kidneys in the New Testament.