Exodus and the Ten Plagues showed the world and Pharaoh who HaShem really is.
Now what was left to do was to show the world who the people of Israel really are.
Indeed the Midrash says that right before the parting of the Red Sea, both nations – the Egyptian as well as the Jewish – were on trial:
!אלה ואלה עובדי עבודה זרה
These as well as those are pagan worshipers!
– was the voice heard amongst the angels in heaven. Because the way the people of Israel lived as slaves in Egypt, and their not having Torah as yet now they were out, made them look, outwardly, little different from their enslavers.
But after the great miracle of the parting of the sea, and the drowning of their enemies, The People and their leader burst into spontaneous song, a praise for Hashem the like of which was never uttered before. It was now that they were proven worthy – of all that was done for them in Yetziat Mitzrayim – worthy of redemption.