Idolatry is the worship of anything other than the One True God of Israel. Just as God's existence is an essential axiom of the Torah that He is one is just as essential. Although God's existence is not really treated in the Torah (because it is assumed), that He is One is. His unity is treated in the Torah mainly because it is so often either misunderstood or perverted by human beings (Laws of the Service of the Cohens and the Customs of the Nations 1:1).
The message of the Torah couldn't be any clearer. There is one God. According to the Rambam:
"For it is the principal object of the Law and the axis round which it turns, to blot out these opinions from man's heart and make the existence of idolatry impossible."
--The Guide for the Perplexed
God is the God of all mankind. We should all strive to know that God is one and totally unique, and remove any notions of internal or external plurality. The Rambam says,
"the actual abolition of idolatry is expressed in the following passage: 'Ye shall destroy their altars, and burn their groves in fire' (Deut. vii. 5), 'and ye shall destroy their name,' etc. (xii. 3). These two things are frequently repeated; they form the principal and first object of the whole Law, as our Sages distinctly told us in their traditional explanation of the words 'all that God commanded you by the hand of Moses' (Num. xv. 23); for they say, 'Hence we learn that those who follow idolatry deny as it were their adhesion to the whole Law, and those who reject idolatry follow as it were the whole Law.' (B.T. Kidd, 40a) Note it."
--The Guide for the Perplexed, 320
Essentially the Hebrew Scriptures teach us that God is one, and nothing else is to be worshiped, even as an intermediary between us and the One God.
God's unity is understood in three parts. First God is alone. Second, God is not physical. Finally, God has a unique identity. Each of these parts must be examined separately.
God is Alone
The Ramchal, Rabbi Moshe Chayim Luzzatto, explained in "The Way of God" that
"It is impossible that there exist more than one being whose existence is intrinsically imperative. Only one being can possibly exist with this necessarily perfect essence, and therefore the only reason all other things have the possibility of existence is that God wills them to exist. All other things therefore depend on Him and do not have intrinsic existence."
--The Way of God, 35
The Ramchal has expressed very beautifully the core understanding of Judaism. God is uniquely One, His existence is necessary because without it nothing else could exist (ibid. 1:1).
Some, such as Hindus, assume an eternal and not-created World "birthed" by another; this is not illogical, but starts with entirely different assumptions that are problematic, since it appears "objectively" that the Universe is not eternal but has a start.
God is clearly alone as attested to by the Hebrew Scriptures.
Deuteronomy 34:35, 39; 32:39; I Samuel 2:2; II Kings 19:19; Isaiah 43:10-11; 44:6-8; Malachi 2:10; and Nehemiah 9:6
This is by no means an exhaustive list.
Any attempt to say that God is accompanied by another god, helped by another, or even that God is in some way internally multiple violates scriptures clear message that God is alone.
Another way to say that God is alone is to say that He is unique any other divine being that exists is God's creation.
Anything that is created is "physical" in the sense that it has otherness from God and in that it is created. But when we think about physicality we usually only think about something composed of matter as being physical.
In the next section we will discuss what we mean by God is not physical.
God is not Physical
Error regarding God's nature often begins by attributing some type of physical existence to God. Such error is often the result of misunderstandings of passages in the Torah such as God sees or stands, or knows.
At times the teachers of Israel have had to correct these misunderstandings. The Rambam in "The Guide for the Perplexed" covers this issue in great detail. According to the Rambam,
"We have stated, in one of the chapters of this treatise, that there is a great difference between bringing to view the existence of a thing and demonstrating its true essence"
--The Guide for the Perplexed, 59
Demonstrating that God exists is different from explaining what God is. Human beings are limited to describing God in ways that make sense to us. We cannot talk about God except for in human terms. Because we can only use human terms to describe God sometimes we forget that these terms are really analogous to God's actions and do not describe God's essence (essence meaning what God is).
"That God exists was therefore shown to ordinary men by means of similes taken from physical bodies; that He is living, by a simile taken from motion, because ordinary men consider only the body as fully, truly, and undoubtedly existing; that which is connected with a body but is itself not a body, although believed to exist, has a lower degree of existence on account of its dependence on the body for existence. That, however, which is neither itself a body, nor a force within a body, is not existent according to man's first notions, and is above all excluded from the range of imaginations."
and he goes on to say,
"...The perception by the senses, especially by hearing and seeing, is best known to us; we have no idea or notion of any other mode of communication between the soul of one person and that of another than by means of speaking, i.e., by the sound produced by lips, tongue, and the other organs of speech. When, therefore, we are to be informed that God has a knowledge of things, and that communication is made by Him to the Prophets who convey it to us, they represent Him to us as seeing and hearing, i.e., as perceiving and knowing those things which can be seen or heard. They represent Him to us as speaking, i.e., that communications from Him reach the Prophets; that is to be understood by the term "prophecy," as will be fully explained."
--Guide for the Perplexed, 60
Human beings use terminology that makes sense to them to discuss God, a being beyond our experience. To talk about God at all means that we must use terms to communicate analogous ideas. Thus terms found in the Tanach that ascribe human qualities such as emotion, physical attributes such as eyes, a body, and limbs, or cognitive functions such as memory, knowledge, and wisdom are meant to be understood to describe God's actions, or inform of God's greatness but creating imagery that instills in us a sense of awe.
Not everyone understands the figurative nature of human attributes to God and thus such individuals mistakenly fall into idolatry. Understanding the simple truth that God is nothing like anything in creation can help us avoid idolatry.
We have already seen from scripture that God is alone, and we have also seen that God is not physical. To put it another way, so far what we have come to understand is that God is unique. His existence is unlike any other existence.
There is another thing that we can say, just as God is unique and has no equal within or beside him, God's essence is unique from every other kind of essence. In other words, God is a unique being. Since we know this it is fitting that the Hebrew Scriptures go on to teach us that not only is God unique, but God's identity is also unique.
In the next section we will discuss God's unique identity.
God has a Unique Identity
God's unique identity is absolutely necessary to know the true God. Although there are religions based (in some way) on Judaism that have claimed to serve the same God as the one professed by the Jewish people, they cannot escape that God has established for Himself a unique identity that is inseparable from the descendants of Israel. This identity is intrinsically connected with the Exodus and Sinai experiences. God is the God of the children of Israel. Although He is the God of all humanity, God's unique identity is as the God of Israel. In giving Israel His Torah, God forever linked His identity with Israel. God tells Israel over and over again "I am your God" (Exodus 6:7; 16:12; Leviticus 11:44-45; 18:2, 4, 30, Ezekiel 34:31, etc.) "my people" (Exodus 5:1; 22:5; Leviticus 26:12; Numbers 24:14).
It is through Israel and their Torah that all nations gain blessing and knowledge of God. Anyone that claims that their god is the same as the God of the Sinai revelation but this god was not known to the Children of Israel at Sinai, or that this god has a different chosen people, or that there is nothing holy about the Torah or that the Torah today is not the same as the Torah of yesterday, or claims that it is not necessary to keep the Torah, this person does not serve the same God of the Jewish people, and has misunderstood something essential about God.
The unique identity of God as the God of Israel is seen clearly from scripture. Exodus 20:3, I Kings 8:60, Hosea 13:4, etc.
In the next section we will discuss other gods.
God addresses Himself to the people of Israel on the issue of the other gods, those that the nations have created for themselves. God makes it clear that His unity is absolute. None of the gods of the nations can make any claim that God can (see Deuteronomy 6:14, I Samuel 2;2, Malachi 2:10, Psalm 81:8-9). Not only that, God makes a stronger claim, that the other "gods" are not real at all, but just images of people's invention.
Now that we have discounted other gods we will talk about the Universal God in the next section.
The Universal God
It is a mistake to think that God is the God of only one particular people. That was the claim of the pagan societies. Every people and culture possessed their gods. The powers of these gods were seen to rise and fall with that of their people. Typically the failure of a god to protect its people from the ill fortunes of war led to the people abandoning their god and serving that of their conqueror. If there is one God only, then He must be the God of not just one people but of all people. God constantly reminds us throughout Scripture that the nations have not been forgotten. They are as much a part of His plan as Israel. The Rabbis teach that the world was created for the Jews so that they could receive Torah, but the Jewish people were created so that they could take that Torah to the world (see I Kings 8:50, II Kings 19:19, Isaiah 45:21-22, Malachi 2:10).
God is unique in Who He is, What He is, and How he relates to human beings. God's existence cannot be compared to any other kind of existence. God is alone having no partner, helper, or internal division. God is uniquely identified as the God of a people—Israel. By attaching His name to Israel he has made identifying the true God dependent upon finding the true people of God. God isn't the God of Israel only, He is the God of all people.