Literally, this means worship of or belief in a single god.
The concept needs to be seen in a broad historical context for its central import in shaping the human condition to be understood.
Essentially all esoteric sources agree that at the root of creation is a sort of unity from which creation in its various branches stems. At some ultimate level of existence which is far beyond human comprehension even in theory, all is literally one.
This concept is not however what is generally understood with monotheism. Monotheism in general usage refers to a personal deity, such as the Yahweh of the Old Testament, the triun God of Christianity or the Allah of islam.
The historically founding concept of monotheism is a covenant between a god and a people. This is a very specific interaction and is quite other than a belief in an eventual unity of all creation. The exemplar of the monotheistic covenant is the deal between Yahweh and Abraham recounted in the Old Testament. Yahweh promises Abraham advantages in return for worship by Abraham and his tribe. This covenant is to be signed in blood, in the form of circumcision of all boy infants on their eighth day. A later story shedding light on the relationship between god and tribe is that of Isaac. Abraham is told to sacrifice his son Isaac to Yahweh and only at the last moment does Yahweh supply a ram to be sacrificed instead. The Lord giveth and the Lord taketh away.
This is not an irrelevant archaic myth. The very same covenant is at the basis of the big three monotheistic religions, Judaism, Christianity and Islam. This approach sets the tone for the rest: God is a jealous God and requires obedience and exclusive worship and will punish the heathen and heretics and reward the blindly faithful. The notion of exclusive possession of a single truth by adherents of any given monotheistic religion creates the notion of the 'other.' This other must be either converted or subjugated, sometimes destroyed.
A claim to exclusive truth is a prime tool for herding masses around a system of authority. This is seen at different scales, from creating tribal cohesion in the stories of the Old Testament to justifying large international acts of aggression, such as the crusades of the Middle Ages or today's escalation between the West and Islam.
On one hand the idea of the jealous God is an accessory to politically or economically motivated aggression or control in general. But monotheism and the idea of a jealous God of exclusivity is not only a tool. It is also a founding principle. This principle posits a sort of divine mandate and entitlement. It is an idea that can lend an unquestionable justification for essentially anything.
Ra tells us concerning the principle of service to self that besides using humanity as a source of etheric nourishment, these same STS entities also wish to recruit and propagate their philosophy. They believe, so Ra says, that this is actually a service to the universe as a whole since they see the path of service to self and contraction as the most efficient way of promoting spiritual development.
This is a sort of battle for souls and cannot proceed by force alone. The manipulated must consent and adopt the ways of the masters. To this effect, these same masters promote the formation of local elites whose allegiance they demand in return for power. Sometimes these entities give power and assistance to the unwary just for creating an idea of chosenness, which will then lead to the formation of a ruling elite within which the doctrine of service to self can take root. In other words, power corrupts.
The whole imposition of monotheism can be seen as a worldwide operation of the above type. Naturally, different factions are needed to promote competition and struggle. On one hand this 'feeds the moon,' on the other it selects the more ruthless elements and provides chances for further polarization to service to self for the 'worthy chosen.'
Based on research more widely discussed in Secret History, it is reasonable to think that the Yahweh of the Old Testament as well as other dominator gods do correspond to actual entities existing at a level between the material and spiritual, occasionally interacting with humanity for their own ends. Despite their claims to the contrary, these are levels below any ultimate creator or unity of all which is.
Many traditions lend support to such a view. For example the Demiurge and archons of darkness of the Gnostics would correspond to this idea. The Absolute III, which Mouravieff equates with the God of the Old Testament also corresponds to the general idea. The 4th Way is rife with references to 'food for the moon' and the 'evil magician.' It is only in character that such a 'god' would claim to be what it is not.
Naturally, there are many ways of promoting elitism and strife for control besides monotheism. Monotheism and dogmatic belief in same simply has a stupendous track record, far better than simple greed or violence for their own sakes. Among the factors that make monotheism a "winner" are that on one hand it frees one from responsibility, can make promises that never need be delivered, can appeal to genuine spiritual questing, can appeal to genuine altruistic motives and self-sacrifice, can claim superior moral status, can appeal to self-importance and vanity, all at the same time. . Monotheism is not the only control system in history but it is a very successful and central one.
In the history of Christianity, we see how a teaching likely promoted by the historical Jesus was co-opted and made an extension of the monotheistic paradigm discussed above. The emperor Constantine shaped the early church from a loosely coupled following of Jesus' and Gnostic teachings into a centralized instrument of power. In so doing, however, a certain amount of the original had to be left in for credibility. Another by-product of this maneuver was that the Old Testament had to be legitimized, bringing its 'Demiurgic' influences into the combination.