We are commanded to know that God is unique and that He alone runs the world. By saying, “Listen, Israel…” the Torah takes this statement from being a mere statement about to Oneness of God to a command to be aware of it. (This is similar to the way, “I am Hashem your God…” in the “Ten Commandments” is not merely a statement that there’s a God so much as it is a commandment to know there’s a God.) The verse that tells us this, which we call the Shema, is so important that we recite it every morning and every evening, as well as on our deathbeds. (We’ll read more about reciting Shema shortly, in Mitzvah #420.)
The reason for this mitzvah should be apparent: knowing that there’s a singular God is the central tenet around which everything else we do revolves. This is so important that one must be willing to perish before denying the Oneness of God. A person who denies God’s singularity is called a kofer b’ikar, one who rejects the fundamental principles of the Jewish faith.
This mitzvah applies to both men and women in all times and places. In the Talmud, it is discussed in the tractate of Brachos on pages 14b-15a, where it is referred to as “kabbalas ohl malchus Shamayim,” “accepting the yoke of the kingdom of Heaven.” It is codified in the Mishneh Torah in the first chapter of Hilchos Yesodei HaTorah. It is #2 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #2 of the 77 positive mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.