Shema Israel - שמע ישראל Why is the Shema so important?

(1) From when may one recite Shema in the evening? From the time when the Kohanim go in to eat their Terumah [produce consecrated for priestly consumption], until the end of the first watch – so says Rabbi Eliezer. And the Sages say: Until midnight. Rabban Gamliel says: Until the break of dawn. It once happened that his [Rabban Gamliel’s] sons came from a house of feasting. They said to him: We have not recited Shema. He said to them: If dawn has not broken, you are obligated to recite it. And [this is true] not only in this case; rather, in all cases where the Sages said that [some precept can be performed only] until midnight — their precepts are [still in force] until the break of dawn. [For example:] Burning the fats and limbs [of the sacrifices, on the Temple altar] — their precepts [can be performed] until the break of dawn. And [another example:] all [sacrifices] which may be eaten for one day — their precepts [of eating them can be performed] until the break of dawn. If that is so, why did the Sages say, "until midnight"? To distance a person from transgression.

(1) Moshe received the Torah from Sinai, and transmitted it to Yehoshua, and Yehoshua to the Elders, and the Elders to the Prophets, and the Prophets transmitted it to the Men of the Great Assembly. They said three things: Be deliberate in judgment, and establish many students, and make a fence for the Torah.