It seems to me that for a nation who has wandered in the wilderness for 40 years, this is a most important and critical question. If the traveling in the wilderness is not done under the supervision of God and is not part of a Divine plan that leads to the land flowing with milk and honey, all of the sufferings of the journey would be for nothing, and this thought breeds despair in the heart of the nation. Immediately, God emphasizes to Moshe and Aharon that a clear miracle must be performed, by which the nation will understand that God is the one giving water to the nation in the wilderness. In other words, the nation suspects that Moshe and Aharon have been leading them of their own accord, and thus, it is important that all of the nation see that God is the one giving water. And, so it was.
Moshe gathers the people before the rock, hits it and out comes water.
However, the main purpose of the lesson is not fulfilled as God desires. From this language, it is possible to understand that Moshe and Aharon have special and supernatural powers, and even wresting water from a rock was within their power - they have the magic and they find the water. Under normal conditions, we would not expect an inaccurate or imprecise wording to engender such a severe punishment. The main educational message God requests to impart to the nation is thwarted, and even more so, the opposite could be infered from the words of Moshe, that indeed he and Aharon have the power. With this in mind, I tried to show that we must read the complaint of the nation in our parasha keeping in mind the background of a parallel journey to Mt. Sinai, and with this in mind, we can try to understand the sin as well.