Ramban

HEAR, O ISRAEL: THE ETERNAL OUR G-D, THE ETERNAL IS ONE. This too is a commandment that offers explanation. For, already contained in the commandment I am the Eternal thy G-d, is [the principle of] the Unity of G-d. As the Rabbis have said: 5. “Rabbi Nathan says: From here there is a refutation to those heretics who say there are two powers governing the universe. For, when the Holy One, blessed be He, stood on Mount Sinai and proclaimed I am the Eternal thy G-d, who protested against Him?” But here he [Moses] came to explain this commandment [I am the Eternal …] and mentioned it after the Ten Commandments because it is the root of faith, and whoever does not acknowledge it denies the essential principle [of the religion] as if he worships idols. “The Eternal our G-d, the Eternal is One. This means: the Eternal, Who is [now, only] our G-d and not the G-d of the [other] nations, will eventually be acknowledged as the One [and only] Eternal, as it is said, In that day shall the Eternal be One, and His Name One.” This is Rashi’s language. Now you must contemplate [the fact] that Scripture changed [the normal usage] here by saying the Eternal ‘our’ G-d and did not state “thy” G-d as it says everywhere else: Hear, O Israel: thou art to pass over the Jordan this day etc. Know therefore this day, that the Eternal ‘thy’ G-d etc.; Hear, O Israel, ye draw nigh this day unto battle etc. for the Eternal ‘your’ G-d is He that goeth with you., 20:3-4. And so also in all sections where [Moses] spoke to Israel he mentioned “the Eternal your G-d” or “the Eternal thy G-d,” and even here [in this very section] he said, And thou shalt love the Eternal ‘thy’ G-d. However, in this declaration of the Unity [of G-d] Moses said, the Eternal ‘our’ G-d because He had done great and awesome things with Moses to make Himself a glorious Name [therefore Moses said “our G-d,” for had he said “your G-d,” he might have appeared to exclude himself from this declaration of Unity]. Now the letter daleth in the word echad (one) is written [in the Torah] large in order to allude to that which is written, He divided the water before them, to make Himself an everlasting Name. And therefore the Rabbis instituted in the reading of the Sh’ma is used as the title for the group of three portions from Scripture (Deuteronomy 6:4-9; 11:13-21; Numbers 15:37-41) which we must recite twice daily, in the morning and in the evening. See “The Commandments,” Vol. I, pp. 15-16. “Blessed be His Name Whose Glorious Kingdom is for ever and ever” and they further said [there]: “This may be compared to a lord’s daughter who smelled the [sweet] odor of some pudding etc.,”, but should be done quietly (Pesachim 56 a). for Moses stated it in the Torah by an allusion. [of the word echad — “one”] which is written large (Bachya). And then he reverted [to the general usage] and stated, And ‘thou’ shalt love the Eternal ‘thy’ G-d like the expression found in [the other] sections of Deuteronomy.