The heart here represents the emotions. Doing something sincerely or completely is spoken of as doing it with all one's heart. Alternate translation: "I will sincerely thank you" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Possible meanings are 1) "in spite of the false idols that exist" or 2) "before the heavenly assembly," which means "in the knowledge of the angels in heaven."