The proof is in the pudding...
"The proof is in the pudding." This is a shortened version of "the proof of the pudding is in the eating." This means that to know something is true you need to experience it yourself. "Is that pudding any good?" "It sure is!" "How do I know what you are saying is true?" "Try it yourself." The prophet Jeremiah is saying that God is making a promise to the people that though their circumstances are dire now, there will come a time when God will set it right. They will experience themselves the truthfulness of God's word to them.
