The Be'er Yitzchak writes: Behold, when David took the five smooth stones to throw at Goliath, who was covered in armor from head to toe as described by the Navi, with what did David come to fight him? Could he overcome him with five smooth stones? What did David think in that act – how could he penetrate the armor and subdue Goliath? Rather, David said: If the iron submits to the stone, though it is an unnatural act, it will merit that Bnei Yisrael use it to circumcise themselves upon entering the holiness of Eretz Yisrael and receiving their name in Yisrael. The reward of the iron for heeding David HaMelech and allowing the stone to penetrate it is that, although the Torah was strict that no iron be waved over the Mizbe’ach, here it is a superior Mitzvah to circumcise with it. The stone that split the iron and pierced the head of the wicked one relinquished its right to perform circumcision and the right to be the means of bringing a Jew into Avraham Avinu’s covenant, as had been the custom until then. It conceded that right to the iron as a reward for its obedience to David! If such a great reward was given to the mute iron for heeding David HaMelech, how much greater is the reward of a person who listens to the voice of the Torah and the voice of Hakadosh Baruch Hu!
The Shulchan Aruch rules (Yoreh Deah, 264:2): One may circumcise with anything, even with a flint, glass, or anything that cuts... and it is a superior Mitzvah to circumcise with iron, whether with a knife or scissors, and the custom is to circumcise with a knife.
