2 Corinthians 3:2-3 (AMPC)

² [No] you yourselves are our letter of recommendation (our credentials), written in your hearts, to be known (perceived, recognized) and read by everybody.

³ You show and make obvious that you are a letter from Christ delivered by us, not written with ink but with [the] Spirit of [the] living God, not on tablets of stone but on tablets of human hearts.