Skepticism

 Rashba (4:234): We learned from our forefathers not to accept something which contains the slightest doubts or uncertainties until it has been thoroughly investigated and the truth is ascertained. This we see concerning the acceptance of Moshe as a true prophet. They were uncertain whether to believe him - even though he came to announce that they were to be rescued from the horrible servitude of Egypt. This is why Moshe said they won’t believe me. This is because it was known that they were inherently skeptical and did not believe anything except that which was unquestionably true. Therefore, even though G d did incredible miracles in Egypt until they were taken out with an outstretched arm and awesome events - it was not sufficient to remove the doubts about Moshe from their hearts. These doubts were caused by the fact that all that occurred in Egypt were possibly just coincidental or natural events or from magical powers. Because of these doubts, they did not have unconditional faith in Moshe until the Splitting of the Sea - as the verse says, “that they [now] believed in G d and Moshe His servant” (Shemos 14:31). The Targum (Shemos 14:31) says they now believed in the prophecy of Moshe that it was true and was not the result of natural events. This event removed the last vestige of doubt that the miraculous events in Egypt could have been the result of random natural events. It was obviously impossible that the sea could have been split at night and the next day return to its normal state. Therefore, the splitting of the sea removed the doubts from their hearts - for the time being. However soon after the Splitting of the Sea, the doubts returned. They thought perhaps Moshe, who was more knowledgeable than any other man had ever been, knew how to do this by natural means which they couldn’t ascertain. The only remaining option for clarifying the truth of Moshe’s prophecy was by their own prophecy and this is what in fact occurred at the Revelation of Sinai when they final established the truth.