A prominent presence at the brit mila is the prophet Eliyahu. In all Jewish communities it is the custom to prepare a special chair for Eliyahu, to greet him, and to place the infant on his chair, as if to receive a blessing from one of our greatest prophets. The source for this custom is in the Zohar.
Eliyahu performed a mighty miracle at Mount Carmel. In a public spectacle witnessed by thousands, a heavenly fire came down and consumed his sacrifice, though it had been soaked in water. The idolatrous prophets of Baal were disgraced and eliminated, and the inspired crowd proclaimed, "HaShem He is G-d, HaShem He is G-d!" (Melakhim I chapter 18). This certainly should have augured the full return of the children of Israel to whole-hearted worship of HaShem.
Yet within days of this remarkable spiritual triumph Eliyahu found himself alone and forlorn in the desert, fleeing for his life from the soldiers of Izevel (Jezebel). After a long sojourn he arrives at Mount Sinai, where he receives a revelation. He complains to HaShem, "I have been very jealous for HaShem, the G-d of Hosts, for the children of Israel have forsaken Your covenant". HaShem replies, "Return, go back on your way".
The Midrash says that HaShem reproved Eliyahu for his statement. No matter how righteous Eliyahu was, and no matter how bitter his experience, he was not justified in criticizing the holy Jewish people, and certainly not in asserting that they had abandoned the covenant (Shir haShirim Rabba on verse 1:6).
The Zohar goes farther and says that HaShem then decreed that Eliyahu will be present at every brit mila that takes place, in order to personally testify that the Jewish people are indeed committed to the covenant. "By your life, in every place that My children will make this holy mark in their flesh, you will be there, and the mouth that testified that Israel abandoned, it will testify that Israel fulfills this covenant." Therefore, we are required to prepare a seat to honor this holy guest and his awesome mission (Zohar Lekh Lekha,I: 93a; Vayigash, I 209b).
To this day, brit mila continues to be one mitzva which is fulfilled by the vast majority of Jews, even those who observe very few laws and customs.
Eliyahu the prophet is present to testify before HaShem that even though many Jews are not careful with particular observances, they are committed to the Jewish people and to our ancient covenant with HaShem, which preceded the commandments.