An appropriate response to someone telling you "I know your heart" would depend on the context and your relationship with that person. Here are a few possible responses, each with a slightly different tone:
Appreciative response: "Thank you for caring and trying to understand me."
Curious response: "Oh, really? What makes you say that?"
Playful response: "Well, my heart can be quite mysterious, so good luck with that!"
Assertive response: "While I appreciate your concern, nobody knows my heart better than I do."
Honest response: "I'm glad you feel that way, but there might still be things about me you don't know."
Polite response: "It's nice to know you think you know me well."
The specific response you choose will depend on your comfort level, the relationship with the person saying it, and the context of the conversation. If you feel comfortable discussing further, you can explore why they think they know your heart or clarify any misunderstandings. If the statement makes you uncomfortable, it's okay to express your boundaries politely. Remember, healthy communication is key to understanding each other better.