Most Jews are not familiar with the biblical Elijah nor with his Talmudic portrayal. They know of him because of childhood memories of the seder — when the Cup of Elijah adorned the table, or when the door was opened in expectation that he would appear. Neither custom has a precise beginning or a clear explanation. Rather, they evolved over centuries; only later did rabbis attempt to explain them.

Both customs are associated with the belief that Elijah will herald the Messiah, and that Israel’s final redemption will take place on the anniversary of the original redemption from Egyptian slavery on the first night of Passover. That anniversary is instituted in the Torah: “It is a night of watch for YHVH, for bringing them out of the land of Egypt; this night is YHVH’s, a watch for all the Israelites through their generations” (Exodus 12:42). Commenting on this verse, a midrash predicts: “On that night they were redeemed, and on that night they are destined to be redeemed.”

The verse in Exodus mentions shimmurim, “watch (or vigil)” twice, both times in the plural. Various rabbis wondered about all this watching. One of them, Rabbi Eliezer, suggested that it implied “a night under constant protection.” On the basis of this guarantee of protection, it became customary to leave the door of one’s home open or unlocked during the first night of Passover.

In some places, opening the door was associated with the declaration near the beginning of the Haggadah: “Let all who are hungry come and eat.” But the 11th-century North African scholar Nissim Gaon associates the open door with Elijah: “I saw that . . . my father would not close the doors of our house….  And until now this is our custom, and [on the night of Passover] the doors of the house are open. When Elijah comes, we will go out to greet him quickly without any delay.”

If Elijah is expected to appear on the first night of Passover to announce the Messiah, won’t he need a cup of wine? So reasoned Zelikman Binga, a 15th-century Ashkenazi rabbi, the earliest known author to mention the Cup of Elijah:

“I have seen some people on the night of Passover who pour a special cup and place it on the table, saying that this is the cup for Elijah the prophet—and I don’t know the reason. But it seems that the reason derives from this: If Elijah the prophet comes on the night of Passover, as we hope and expect, he too will need a cup, for even a poor person among Israel must drink no less than four cups.”

Some linked Elijah’s cup with the declaration near the beginning of the seder: “Let all who are hungry come and eat; let all who are needy come and celebrate Passover.” As one author noted, “Since one calls for ‘all who are needy’ to ‘come and eat,’ he should prepare a cup for a guest who may come; and they call that cup ‘the Cup of Elijah the prophet,’ because we hope for this guest.”

A more utilitarian explanation was offered in the 17th century by the Sephardic authority Hayyim Benveniste, who reports:

“This is the custom I saw among a few Ashkenazim: to leave on the table one empty cup . . . in which to pour all the wine left over in the cups of all those reclining there [after they have drunk the required minimum]. This cup is called the Cup of Elijah the prophet (gratefully remembered).”

Among the various attempts to explain the Cup of Elijah, one relates to how many cups of wine a person is required to drink at the seder. The standard practice is to quaff four cups. But according to early manuscripts of the Talmud, Rabbi Tarfon mentions a “fifth cup.” Elsewhere in the Talmud, we are told that one day Elijah will come and resolve all halakhic disputes. Presumably, he will then determine the status of this additional cup, so the cup is appropriately named for him! This explanation was attributed to another famous Elijah — Elijah ben Solomon, the Gaon of Vilna.

On Passover, Elijah is expected to appear on the threshold. Anticipated at the doorway, he mediates between home and community, between private space and the wider world. He links the ancient liberation from Egyptian slavery with messianic deliverance, bridging the chasm between this unredeemed earth and the final redemption.

But memories of Elijah and the seder can be bittersweet, because of the annual disappointment of not finding him at the door. Once, before Passover, according to a Hasidic tale, the disciples of the Kotsker Rebbe complained to him about this. He promised them that Elijah would be revealed to them at the upcoming seder. On the first night of the festival, the room was full, the atmosphere charged, with Elijah’s cup waiting on the table. The seder proceeded, and finally the door was opened. What happened next left the disciples astonished. Nothing; no one appeared.

Crushed, they turned to their Rebbe, whose face was beaming. Seeing their distress, he asked, “What’s troubling you?” They told him. “Fools!” he thundered. “Do you think Elijah the prophet enters through the door? He enters through the heart.”

This piece is an excerpt from Daniel C. Matt’s new book, “Becoming Elijah: Prophet of Transformation,” winner of the inaugural Rabbi Jonathan Sacks Book Prize. His next series of online Zohar courses begins on April 25.