In every generation, we are required to view ourselves as having personally left Egypt, as it is written: You shall tell your child on that day (even many generations after the Exodus), "It is because of this that G-d acted for me when I left Egypt." It was not only our ancestors that the Holy one, blessed be He, redeemed from Egypt. He redeemed us as well along with them, as it is written: It was us that He brought out of there, to bring us to and give us the land that He promised to our ancestors.
A seer tells his friend to flee the country because of an impending war. The man follows this advice, emigrates, and raises many generations of offspring. One would not refer to the seer as having saved all of his friend's future offspring. Rather, the seer saved the patriarch, and this salvation resulted in the possibility for future offspring.
Why then do we say that G-d redeemed all generations?
The answer is that when G-d redeemed us, He implanted a sense of freedom in the souls of our people for all generations. Thus, even when we are persecuted, we remain internally free.
This is why it was G-d Himself who carried out the salvation--not an angel--since only G-d could have affected this eternal transformation of the Jewish soul.
So our freedom today is not only the result of our ancestor's freedom but because of the freedom we were granted during the Exodus.