Throughout our long history the Jewish people have zealously guarded the mitzvah of circumcision, even during periods of religious rebellion and apostasy. Only once was there any weakness in the fulfillment of this mitzvah. It was after King Solomon's death, approximately five hundred years after the Jewish people entered the Land of Israel.
During this period the nation was divided into two kingdoms. A Jew by the name of Ahab was appointed king over ten of the 12 tribes. During his reign he established harsh decrees forbidding the Jewish people from following the Torah and performing its mitzvot, including the mitzvah of circumcision.
Elijah the Prophet, seeing the Jews turning away from God and brazenly disregarding the mitzvah of Brit Milah, proclaimed that Heaven should withhold its rain and dew from the earth until the Jews would repent. The wife of Ahab, Queen Jezebel, was enraged by his actions and sentenced Elijah to death.
Elijah turned to God and prayed for protection. God said to him, "Are you different than your ancestors? When the patriarch Jacob was threatened by his brother Esau he fled from him. Similarly, when Moses was in danger from Pharaoh he fled from him to Midian. And when King David was threatened by King Saul he also fled from him." Elijah took God's words as a sign that he must also flee, and he fled to the mountain of Choravah.
God revealed himself to Elijah and said, "Elijah, why are you here?" Elijah replied, "I was zealously protecting Your honor." God replied. "You are constantly indignant. You were angered in the wilderness when the Jews began marrying the women of Moab. Now you are again righteously angered by seeing My children neglecting the commandment of circumcision. I therefore swear by your life that you will witness My people perform the mitzvah of circumcision for all eternity!"
Elijah then said, "God, You know that I am easily aroused to zealously uphold Your honor. Thus, when I am in the presence of anyone who has transgressed even one of Your holy commandments I will not be able to remain quiet!" God then promised Elijah that on the day of the Brit the father of the child, the Mohel, and all those present will be completely forgiven of their sins. (From Midrashic sources)
This is the reason why at every Brit there is a special chair designated for Elijah the Prophet.