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.
Moses’s three questions in chapter 4 expose Moses’s fear and unbelief. Yet for each of Moses’s three objections, God patiently brings himself back into Moses’s thoughts. Moses was very hesitant to obey God’s call. All of us can be the same way. The point is this, we must submit to God’s will. In big things, in little things, in hard things, and in difficult things, we must lovingly submit to the will of God. In this passage, there are three reasons why we should submit to God.