Although I have my qualms about the particular way that OP has formulated the question, it is an interesting one and could be sharply focused on two texts -- or textual clusters -- in the Hebrew Bible.
Terms must be distinguished; I would do it this way:
- polytheism is the belief that there are many gods who require devotion;
- henotheism is the belief that there is one particular god to whom alone devotion is given, although there are other gods;
- monotheism is the belief that there is only one god (and no others), and devotion is accorded to this god;
- atheism is the belief that there are no gods, and consequently no devotion to be rendered.
Using these terms consistently helps in looking at how the Hebrew Bible formulates its reference to divinity. We are mostly dealing with the question of which of the first three definitions most accurately represent the texts under discussion.
1. The Shema
The "Shema" (pronounced "shma") is the confession of Deuteronomy 6:4, already discussed in an answer provided. I will only add here, then, that translation of this confession is a vexed issue. A helpful discussion of the issues -- and one suggested solution -- is found in R.W.L. Moberly, "'Yahweh is One': The Translation of the Shema", in Studies in the Pentateuch ed. by J.A. Emerton (Brill, 1990), pp. 209-215 (unfortunately, not wholly available in Google Books "preview").
It should be noted, though, that the formulation of this confession is not a technically a "monotheistic" confession: that is, its grammar/syntax leave open the question of precise "monotheism". It could, for example, be a confession that this God only is the God of Israel -- thus leaving open the question of whether there are "other" gods. Note, for example, how Rashi's comment inclines to exclusivity of this God to Israel, rather than to considerations of the existence of one or many gods.
That is, although the Shema may admit of a monotheistic understanding and is at home in a monotheistic framework, strictly linguistically speaking, it is a henotheistic confession. To see monotheism (as defined above) in the Hebrew Bible, we need to look elsewhere.1
2. The Book of Isaiah
If monotheism as defined above is to be found anywhere in the Hebrew Bible, the most likely place is in certain formulations found in the book of Isaiah, coming to clearest expression in Isaiah 45:21-22 [NRSV]:2
21 Declare and present your case;
let them take counsel together!
Who told this long ago?
Who declared it of old?
Was it not I, the Lord?
There is no other god besides me,
וְאֵין־עוֹד אֱלֹהִים מִבַּלְעָדַי
wəʾên ʿôd ʾĕlōhîm mibbalʿāday
a righteous God and a Savior;
there is no one besides me.
22 Turn to me and be saved,
all the ends of the earth!
For I am God, and there is no other.
כִּי אֲנִי־אֵל וְאֵין עוֹד
kî ʾănî ʾēl wəʾên ʿôd
Similar language is found also in Deut. 4:35, 39; 1 Ki. 8:60; Isa. 45:5, 6, 14, 18, 21, 22 (as noted); and 46:9. Labuschagne (see note 2) discusses some other texts that resonate with this language as well.
Even these formulations admit of other interpretations, and some scholars resist seeing the Isaiah texts as expressions of "full blown" monotheism, or see them rather as only incipient monotheism. For others, however, this language which excludes the possibility of any other god than YHWH is as clear an expression of monotheism as the Hebrew Bible provides.
- For a recent and careful consideration of this issue, see N. MacDonald, Deuteronomy and the Meaning of "Monotheism" (Mohr Siebeck, 2012).
- One major study of this language is found in the monograph by C. J. Labuschagne, The Incomparability of Yahweh in the Old Testament (Brill, 1966).