If we were to synthesize the myriad of topics found in the book of Exodus, we could conclude that the two most important messages that we get out of the book are the following:
The Israelites are the chosen people of God:
The God of the Israelites is the god of Moses, as well as the god of Abraham. His name is "I AM" (or "I AM who I AM", also, Yahwe, Jehova, et al), and he declares himself as the liberator and protector of his people. To the modern reader this may sound obvious, but we are talking about a writing created during a time when these tenets were imperative to the Israelites as confirmation of themselves as a nation.
Coming straight from slavery into the possibility of a new land, the people of Israel needed several forms of validation to begin their journey. Having the validation of God means that they could move forward as an independent nation. The final confirmation of God's commitment to the people of Israel is sealed with the final covenant between themselves and God, found in Exodus 20 - 23 and 25:12. From that moment on, Israel would be God's covenant-nation, ruled by the holy principles found in the Commandments.
The second most important theme found in Exodus could clearly be:
God is Almighty.
Exodus illustrates in several occasions how the God of the Israelites "I AM" is quite almighty. It is in this book where we see their God manifest most furiously in the most dedicated fashion. First, God assists in the liberation of the Israelites from Egypt by giving 10 divine plagues to the Egyptians. This is proof of the wrath and power of a God that demands that his people are freed. These plagues were, in order a) the turning of water into blood, b) infestation of frogs, c) gnats, d) swarms of flies, e) death to the livestock f) boils, or bubos, on the skin of the Egyptians, g) hail, h) locusts, i) darkness, and the worst of all: the death of every firstborn in Egypt (7:19-11:5)
Also, God does the well-known parting of the Red Sea to allow the passage of the people. Moreover, He also provides for the people light and guidance throughout their journey.
The Lord was going before them in a pillar of cloud by day to lead them on the way, and in a pillar of fire by night to give them light, that they might travel by day and by night. He did not take away the pillar of cloud by day, nor the pillar of fire by night, from before the people (Exodus 13:21-22).
He also provides them manna for their survival in the desert (16:35), and protects them even in times of weakness from the part of the people.
In all, God's confirmation of his purpose with the people of Israel, as well as the clear proof that He is an Almighty God, are the two most important themes that come out of the many events that take place in the book of Exodus.