Why pour a cup if we lack the ability to drink it?
In the course of the Passover Seder we drink four cups of wine, corresponding to the four "expressions of redemption" in the Divine declaration (Exodus 6:2-8):
"I will take you out",
"I will deliver you",
"I will redeem you",
"I will acquire you."
For the Exodus incorporated in itself four distinct redemption’s: our physical removal from the geographical boundaries of Egypt ("I will take out"); our delivery from Egyptian hegemony ("I will deliver you from their bondage"); the creation of an inherently free people, immune to any future possibility of enslavement ("I will redeem you"); and our election as G‑d's chosen people at Sinai seven weeks later on the festival of Shavuot ("I will acquire you as My nation, and I will be to you a G‑d") - the purpose and goal of the Exodus.
In truth, there is also a fifth "expression of redemption" in G‑d's communication to Moses - the promise that "I will bring you into the land." Indeed, there is a corresponding fifth cup of wine - the "Cup of Elijah" - that is filled during the final stage of the Seder.
But this cup of wine is not drunk; instead, it is placed at the center of the table where the children keep watch over the gently quivering liquid, hoping to detect a sign of its sampling by Elijah the Prophet, in whose honor it was poured.
The first four elements of the redemption are something that we are to "drink" - to actively pursue and realize ourselves.
It is within our power to overcome all that limits and enslaves us, both physically and spiritually, both without and within; to develop our potential for freedom, and to exercise this freedom as the freedom to fulfill our mission as G‑d's people as communicated to us at Sinai.
But the final and culminating level of redemption - its "I will bring you" element, which shall be fully realized only in the era of Mashiach - is something that transcends our human efforts.
This is not a cup we can drink on our own. We can only bring ourselves to the threshold of this Divinely perfect world, through our active realization of the first four "expressions of redemption."
The drinking of the fifth cup awaits Elijah, herald of the final and ultimate redemption.