Sforno

ADONAI APPEARED TO HIM BY THE TEREBINTHS OF MAMRE. Because it was there that Avraham was circumcised at that time, and his whole household were circumcised with him, it was there that God, may He be blessed, showed His presence to His people in covenant, according to the practice of all those who enter covenants, as in (Deut. 19:9-11), "You are standing ... to enter into the covenant, and as in (II Kings 23:3), "He entered [the covenant] ... before Adonai ... and all the people entered into the covenant." And He appeared to Avraham, who was the most prepared for this vision. Similar is the matter of (Exod. 4:24), "Adonai encountered him" -- He did not appear there in order to talk with Moshe, but He made His presence appear in order to receive the covenant of his son, as He said (Gen. 17:11-12), "between Me and you throughout the generations." And perhaps it is because of this that there is a custom to set up a chair at the time and place of a circumcision.