Hashem commanded the Red Sea to spit out the Mitzrim onto its banks in order that Bnai "Yisrael would not say, "Just as we went up on this side they went up on another side further from here and will pursue us later."(Rashi) We see from here that Bnei Yisrael did not believe they were worthy of Hashem's promise to be redeemed. When they saw their enemy completely destroyed they no longer looked at themselves as fleeing slaves. At this point they believed that Hashem's promise of redemption had come true and that Moshe was sent as the Redeemer.
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