Exodus 15:22
So Moses brought Israel from the Red sea, and they went out into the wilderness of Shur; and they went three days in the wilderness, and found no water. So Moses brought Israel from the Red sea, and they went out into the wilderness of Shur; and they went three days in the wilderness, and found no water.
After deliverance at the Red Sea, Israel entered the wilderness of Shur and traveled three days without finding water. The waterless march brought hardship and thirst, and the people murmured against Moses. The road led on to Marah, where the water was bitter and unusable, increasing rather than quenching thirst. This difficulty tested their trust in God.
At Marah the test bore on trust in the Lord as the invisible leader and in Moses as the visible leader. God permitted the waterless journey and the bitter water to test whether they had learned trust and obedience from His deliverances in Egypt and at the Red Sea. He sometimes brings His people into difficult places so He can reveal His power and glory by delivering them. With Marah and Elim close at hand, the verse shows hardship and relief following one another, for God sometimes makes bitter water sweet and sometimes provides pleasant springs and shade. The desert journey pictures earthly life, where grief and joy lie close together and both hardship and comfort pass before the final rest in God's city.