19 Before this I had been like a docile lamb ready to be led to the slaughter. I did not know they were making plans to kill me. I did not know they were saying, “Let’s destroy the tree along with its fruit! Let’s remove Jeremiah from the world of the living so people will not even be reminded of him anymore.” 20 So I said, “O Lord of Heaven’s Armies, you are a just judge! You examine people’s hearts and kidneys57. I want to see you pay them back for what they have done because I trust you to vindicate my cause.” 21 Then the Lord told me about some men from Anathoth who were threatening to kill me. They had threatened, “Stop prophesying in the name of the Lord or we will kill you!”
22 Look! I am throwing her onto a bed of violent illness, and those who commit adultery with her into terrible suffering, unless they repent of her deeds. 23 Furthermore, I will strike her followers with a deadly disease, and then all the churches will know that I am the one who searches kidneys* and hearts. I will repay each one of you what your deeds deserve. 24 But to the rest of you in Thyatira, all who do not hold to this teaching (who have not learned the so-called “deep secrets of Satan”), to you I say: I do not put any additional burden on you.
Notes and References
"... God wants the churches to know ὅτι ἐγώ εἰμι ὁ ἐραυνῶν νε ροὺς καὶ καρδίας (hoti egō eimi ho eraunōn nephrous kai kardias, that I am the one who searches the feelings and the thoughts). When Christ “searches” a person, he makes a critical investigation that lays bare the deepest part of a person’s being. The language is reminiscent of Jer. 17:10, “I the LORD search the heart and examine the mind.” The νε ρός is literally the “kidney,” in Hebrew thought the core of emotions and feelings. The καρδία is the seat of the intellect and mind. Together they constitute one’s inner being. Nothing can be hidden from the inquiry of the Lord. Therefore, “all churches” had better examine their own house, lest the same judgment befall them, for they will not get away with their sins ..."
Osborne, Grant R. Baker Exegetical Commentary: Revelation (p. 133) Baker Academic, 2019