The Zohar, in Parashat Lech-Lecha (listen to audio recording for precise citation), writes that when a child is brought to his Berit Mila, the Almighty calls to His entourage of angels and exclaims, "Look what My creatures are doing in the world!" At that moment, the Zohar continues, Eliyahu Hanabi arrives to attend the Berit. He must fly four "flights" in order to reach the place of the Berit, corresponding to the four letters of the divine Name of "Havaya."

Since Eliyahu attends every Berit Mila, a special chair must be designated for him at the Berit, in addition to the chair on which the Sandak sits. The Zohar emphasizes that the chair must be verbally designated as Eliyahu’s chair, as otherwise, the special spirit of sanctity is not present at the event. It is therefore customary to declare before the circumcision, "Ze Kiseh Shel Eliyahu Mal’ach Ha’berit" ("This is the chair of Eliyahu, angel of the covenant"). Our community also follows the custom of placing on the chair a Humash opened to Parashat Pinhas, as the Sages teach that Pinhas was Eliyahu.

The reason why Eliyahu attends every Berit, the Zohar explains, is because he said to Hashem, "I was zealous for Hashem, Lord of Hosts, for Beneh Yisrael abandoned Your covenant" (Melachim I 19:10). God reacted angrily to Eliyahu’s disparaging remarks about His children, Beneh Yisrael, and therefore ordered Eliyahu to personally attend each and every Berit Mila from that time on, to see that the Jewish people have remained committed to their covenant with God. And after the Berit, Eliyahu must return to God to testify that the Berit was performed.

Summary: Eliyahu Ha’nabi’s chair at a Berit Mila should be verbally designated as his chair. Our community also follows the custom of placing on the chair a Humash opened to Parashat Pinhas.