This month, Nisan, shall be for you the beginning of the months; it shall be for you the first of the months of the year.
Until this point in history, Tishrei--the month of creation--had been considered the first month of the year. Now, although the years continued to be counted from Tishrei, Nisan became the first month and Tishrei became the seventh.
This is because until the Exodus, the world was under the influence of the constant Divine forces that G-d set in motion as the creation of the world and that we call "nature." True, there had been occasional miracles, but these were the rare exception rather than the rule. It was therefore fitting to consider Tishrei the primary month, since the forces of nature that govern the world came into being in that month.
The Exodus, in contrast, heralded the birth of the Jewish people, who would not be subject to the limitations of nature. By virtue of their connection to G-d and His Torah, the Jewish people, both individually and collectively, would be living, walking miracles. The Exodus itself was the first, dramatic example of this freedom; the mass release of a whole population of slaves from the world's preeminent superpower proved to an awestruck world that his people was connected to a power that does not answer to the laws of nature. The survival of the Jews throughout history, despite all odds, further testifies to their defiance of natural law. But the most profound way in which the Jewish people transcends nature is by achieving, via the Torah, transcendent Divine consciousness--an awareness of G-d and connection to Him that is totally out of context in this finite, nature-oriented world.
Therefore, Nisan, the miracle-month (the word Nisan means "miracle") now became the primary month. True, nature still exists, and therefore the years are sill counted from the first of Tishrei, which remains Rosh Hashanah, t he day on which the world's material sustenance for the coming year is determined. But in terms of purpose, Nisan from now on assumes pride of place, for the purpose of creation is to infuse the natural order with
G-d's transcendent presence through the Jewish people's dedication to the Torah and its commandments.