Grumbling is the language of unbelief. They were offended not by the claim being false, but by the claim being too big to fit their small picture of who Jesus could be.
Memory phrase: Don't grumble — believe.
Talk about it: Have you ever grumbled about something instead of asking a question? What happened?
Prayer: Lord, when something You say is hard to understand, help me ask questions with a humble heart instead of grumbling.