The people of Israel struggled to see the true God of Israel that had rescued them from Egypt. Their pattern of grumbling continued to spiral as each time they became more bitter and frustrated with Moses. But the Lord would show them a glimpse of what their grumbling would lead to: a substitutionary atoner.
"Behold, I will stand before you there on the rock at Horeb, and you shall strike the rock, and water shall come out of it, and the people will drink." Exodus 17:6