The 2nd mitzvah is that we are commanded to acquire knowledge1 of the nature of G‑d's Unity, i.e. to understand that the Original Creator and Source of all existence is One.

The source of this commandment is G‑d's statement2 (exalted be He), "Hear [i.e. "understand"] O Israel, G‑d is our Lord, G‑d is One."

In most Midrashim you will find this mitzvah described [in the context of G‑d's statement that a certain kindness was done to the Jewish people,] "on condition that they unify My Name," "on condition that they unite Me," or a similar expression. They mean to say that G‑d took us out of bondage and heaped kindness upon us upon condition that we have His Unity firmly fixed in our minds — since we are required to do so. [From this we see that knowledge of His Unity is an actual requirement, and is therefore counted as one of the 613 commandments.]

In many places the expression is used, "the mitzvah of His Unity" [the word "mitzvah" also indicating that this counts as one of the 613 commandments.]

Our Sages3 also called this mitzvah "Kingdom," saying that [the paragraph Shema is read before V'haya] "in order to accept upon oneself the yoke of the Kingdom of Heaven," i.e. to acknowledge and comprehend His Unity.4