en_tn/1co/07/37.md

But if he is standing firm in his heart

Here "standing firm" is a metaphor for deciding something with certainty. Here "heart" is metonym for a person's mind or thoughts. Alternate translation: "But if he has decided firmly in his own mind" (See: rc://en/ta/man/jit/figs-metaphor and rc://en/ta/man/jit/figs-metonymy)