What is "Romemos Kel bi'Gronam"?
Brachos 5a: This is when they recite Shema before sleep. We learn from the previous verse "Yalzu Chasidim b'Chavod Yeranenu Al Mishkevosam."
Radak: When they will go to fight Gog and Magog, they will pray to Hashem, and exalt Him for taking them out of Galus. So He will give to them strength to eradicate their enemies. Voice starts in the Garon (throat), and finishes in the mouth. Alternatively, they will scream in their Tefilah - "Kra v'Garon" (Yeshayah 58:1).
Malbim: It is rebuke of the nations. Through Hashem's grandeur, they will debate with the other creeds and clarify the truth of their Emunah, via showing the grandeur of Kel and His Gevurah.
What is "v'Cherev Pifiyos"?
Radak: It is a sword in which both edges of the blade [are sharp, and] kill.
What do we learn from "Romemos Kel bi'Gronam v'Cherev Pifiyos b'Yadam"?
Brachos 5a: Anyone who reads Shema before sleep (refer to 149:6:1:1) is like one who bears a double-edged sword 1 (to kill Mazikin).
Rashi: The exalting of Hashem in their mouths is a double-edged sword in their hands.
Radak, Malbim: [At the time] they pray with their mouths, they will hold a double-edged sword in their hands.
Rema (YD 265:11): This is the source for the custom that a Mohel is Shali'ach Tzibur on the day of the Bris.
Ma'alos ha'Torah (DH uv'Zohar ha'Kodesh Parshas va'Eschanan): Keri'as Shma has all kinds of war [weapons], like it says in the Zohar.
