Here "hearts" represents a person's mind. Alternate translation: "think to themselves" or "say to themselves" (See: [[rc://en/ta/man/jit/figs-metonymy]])
The abstract noun "strength" can be stated as "strong" or "encourage." Alternate translation: "make us strong" or "encourage us" (See: [[rc://en/ta/man/jit/figs-abstractnouns]])