What is the central message of Pesah? Is it the removal of hametz from our homes and our hearts? Is it the requirement that in every generation we should see ourselves as having personally come forth from slavery to freedom? Is it the celebration of the spring wheat harvest?
Contextually speaking, the central message is the following:
הָא לַחְמָא עַנְיָא דִי אֲכָלוּ אַבְהָתָנָא בְּאַרְעָא דְמִצְרָיִם. כָּל דִכְפִין יֵיתֵי וְיֵיכֹל, כָּל דִצְרִיךְ יֵיתֵי וְיִפְסַח.
"This is the bread of poverty that our ancestors ate in the land of Egypt. Let all who are hungry come and eat; let all who are in need come and take part in the Pesah festival."
Three observations point to the centrality of this message: (1) it's found right up front in the fifth step of the seder, or order, that we go through on the first two nights of Pesah, just after the Four Questions. (2) We read this after having drunk only one cup of wine. And (3), unlike the bulk of the haggadah, it's not in Hebrew - it's in Aramaic, which is the language that the Jews spoke when the seder originated.
This was the message that the ancient rabbis wanted us to take away from Pesah - that it is our duty to remember those that are hungry, even as we sit down to eat a huge, festive meal. Freedom from slavery means that just as we must procure our own food, so too must we see to it that all are fed.
Why, then, do we open the door AFTER dinner, ostensibly to welcome the prophet Elijah? The proper time to open the door is right at the beginning, to demonstrate that we mean it when we say, "Let all who are hungry come and eat."