The Zohar Explains...

Text from:

http://www.ascentofsafed.com/cgi-bin/ascent.cgi?Name=ten

Quoting from the above link:

"The 1st (commandment, instructing us to have faith in G-d) {which was given at Mount Sinai} states: 'I am the Lord your G-d...' Regarding creation, the verse {the 1st imperative of the word of G-d} states: 'There shall be light, and there was light.'"5

NOTES

Footnote #5 from this quote (which I have copied below) seems to distinguish between “Bereshit” as a commandment and “Let there be light” as a commandment. 

There is a parallel between the manner in which the obedience of faith in G-d is indicated in the first commandment given to Israel at Mount Sinai and the manner in which the expression of G-d’s word in the first word of Torah,“Bereshith” precedes the formal imperative of “Let there be light.” Although this is true, in neither case is there a true separation of statement which can be semantically distinguished.   The "Bereshith" of Genesis 1:1 flows semantically into the "Let there be light" of Genesis 1:2.  And the "I am the Lord your God..." flows semantically into the imperative form, "You shall have no other gods before me..."  In both cases there are two commandments but the imperative is only implicit in the first and made explicit in the second.

The first commandment is whole and comprehensive in both cases, so that the second and subsequent commandments flow out of the first.   This organic wholeness of the first commandment in both instances reflects, first of all, how that in Adam’s creation there was no need for an explicit imperative to believe in G-d but that it was written in their nature.   The words, "Bereshith Elohim..." were indeed a commandment to Adam to believe in G-d, though not explicit because it was a commandment written in Adam's very existence and in their very lifeblood.  When it was made explicit as an imperative in "Let there be light", it was possible, therefore to conceal this light when it became necessary, so that it was not necessarily heard as a direct commandment to Adam.  

And yet on Mount Sinai it was necessary that this same word that was originally written in the nature of Adam by creation be made an explicit imperative to Adam directly.  Not even the clause, “who brought you up out of the land of Egypt, out of the house of bondage,” was missing in an appropriate form from the original “writing” of this commandment on the heart of Adam by nature of their creation. 

For, this clause “written” in Adam’s nature was actually alluded to in the formalized imperative that was given to Adam, “Do not eat of the fruit of the Tree in the midst of the garden.”  This formalized commandment given in the garden has a direct correspondence with the second commandment given to Israel.  And just as the second commandment in the Ten Commandments alludes directly to and follows directly from the first commandment, so does the commandment not to eat of the fruit of the tree in the midst of the garden allude to the commandment written by nature in Adam’s heart and nature to believe in and obey G-d. 

In other words, implicit in the commandment “written” in Adam’s nature to believe in G-d was the possibility of two different conditions of Adam, Adam alive or Adam dead, Adam with God or Adam without God.  Now in the Covenant Made New it is written of the nations, (which is to say, Egypt), that they were without G-d in the world (Ephesians 2:12).  But it is to be understood from this statement that Israel, on the other hand, ever since G-d had placed His claim upon that nation by means of the prophetic promise of the Passover lamb and the redemption from Egypt, was not without G-d in the world. 

The nations, therefore were categorically in the condition of Adam being spiritually dead, but Israel was categorically in the condition of Adam being spiritually alive, even though any or many of the children of Israel themselves might not believe in and obey G-d.  Israel was categorically in the condition of Adam with G-d, Adam living.  But it was by virtue of the promise of the eternal redemption of Israel through the blood of Mashiach, the true Passover Lamb.  Therefore G-d could - and it was necessary for G-d to - reiterate more explicitly to Israel the first commandment, which originally had been “written” in the heart and in the nature of Adam.  It was necessary that it should be reiterated more explicitly because now it was a commandment of repentance.

Still, though the commandment to believe in G-d became more explicit to Israel than the commandment to believe in G-d had been to Adam, it was not made explicit as a formal positive imperative itself, but instead still borrowed from the second commandment, which flowed out of it, the imperative form.   This is because the first commandment is a reiteration of "Bereshith" and not an entirely new or different commandment.  It is still a commandment that remained written in the very existence and lifeblood of Adam, which is to say, Israel.