This is what you must say in your heart

Here "heart" represents the thoughts of a person. AT: "You ask yourself" or "You must say to yourself" (See: rc://en/ta/man/translate/figs-metonymy)

How will we recognize a message that Yahweh has not spoken?

"How will we know if the message that the prophet speaks is from Yahweh?" Here "we" refers to the people of Israel.