The real force of the story is unveiled in the second part of v 25: There the LORD made for them a statute and a rule, and there he tested them. The LORD was leading his people. He was the one who led them to Marah. The LORD was the one who, in their desperate need for water, brought them first to the bitter waters. But v 25 is the divine perspective on these earthly affairs. Marah was a test. The Lord was trying his people to see if they would trust him. Thus the lesson from God’s Word is, in times of great difficulty, we must trust the Lord.