Matthew 21:22 (New International Version)
I am sure the children of Israel had no doubt that God is great and mighty and nothing is impossible with God. They had seen how God worked the ten plagues to bring them out of Egypt. They had seen the parting of the Red Sea, a miracle which happened just days before they came to Marah. When we read the story of Bitter Water at Marah, we wondered why they never thought that the God who parted the Red Sea and delivered them from Pharaoh would certainly never leave them to die in the wilderness.
Click here to read why the children of Israel were having difficulty believing.