In our time monotheism is often considered a higher form of religion than polytheism. People speak of God, pray to God, search for God rather than speak of gods, pray to a particular god, or acknowledge various divine incarnations. Individual monotheistic worshipers, however, usually worship a particular form of their god and not his causal, unmanifest, formless aspect. There is a nearness, a response, in the formal aspect which is lacking in the abstract conception. But a causal, formless, all-pervading divinity, cause and origin of all forms, cannot be manifest in a particular form and would of necessity be equally at the root of all types of form. Divinity can only be reached through its manifestations, and there are as many gods as there are aspects of creation. The gods and the universe are two aspects -- the conscious powers and the unconscious forms -- of an indefinite multiplicity.

         In the polytheistic religion each individual worshiper has a chosen deity (ista-devata) and does not usually worship other gods in the same way as his own, as the one he feels nearer to himself. Yet he acknowledges other gods. The Hindu, whether he be a worshiper of the Pervader (Visnu), the Destroyer (Siva), Energy (Sakti), or the Sun (Surya), is always ready to acknowledge the equivalence of these deities as the manifestations of distinct powers springing from an unknown 'Immensity.' He knows that ultimate Being or non-Being is ever beyond his grasp, beyond existence, and in no way can be worshiped or prayed to. Since he realizes that other deities are but other aspects of the one he worships, he is basically tolerant and must be ready to accept every form of knowledge or belief as potentially valid. Persecution or proselytization of other religious groups, however strange their beliefs may seem to him, can never be a defensible attitude from the point of view of the Hindu.

         From the vast and solid basis for experience formed by the multiplicity of divine manifestations the polytheist can rise toward the goal beyond reach that is nondualism and toward the illusion of an ultimate identification. At every step he finds within the multiplicity a lesser degree of differentiation suitable to his stage of development as he travels from the outward forms of ritual and morality toward the more abstract aspects of knowledge and nonaction. These are outwardly represented by different groups of static symbols, that is, deities, and active symbols, that is, rites. The seeker chooses at each stage the deities and rites which are within his reach as he progresses on the path that leads toward liberation.

         During the pilgrimage of life he goes from one temple to another, adopts different forms of ritual, different modes of living, and various means of self-development. He is constantly aware of the coexistence of different approaches to divinity, suitable for people at stages of realization different from his own.

         It is considerably more difficult, within a monotheist creed, for an individual to establish the hierarchy of his attitudes to divinity at different stages of his development, and it is almost impossible for him not to mix planes and methods, for relative truth is different at each stage and yet its thorough understanding is essential if a particular stage is to be outgrown.

         Since he cannot clearly see side by side, illustrated in different symbols, in different cults or philosophies -- and in the attitude of their followers -- the different stages of his own development, past as well as future, any attempt at looking beyond the limits of his creed makes the monotheist lose his balance. It is because of this precarious equilibrium that, in monotheist creeds, we find so little room between proselytizing and irreligion, so little place for tolerance, so little respect for modes of thought, or worship, or behavior different from the 'norm.' The monotheist, as a rule, confuses the religious and the moral planes, conventional practices with self-development. He mixes up faith with proselytism, mystical emotion with spiritual progress.

         The man who finds himself at a stage of development different from that for which a given monotheistic system was devised has hardly any alternative but to abandon it, which often means, if he has no contact with other religious forms, abandoning religion and spiritual search altogether or devising some system of his own unlikely to lead him toward modes of thought and understanding of which he has not already an idea.

         Monotheism is always linked with a culture, a civilization. It is not through its forms but in spite of them that gifted individuals may reach spiritual attainment. We shall see that monotheism is the projection of the human individuality into the cosmic sphere, the shaping of 'god' to the image of man. Hence the monotheist commonly visualizes his 'god' as an anthropomorphic entity who shares his habits, patronizes his customs, and acts according to his ideals. Religion becomes a means of glorifying his culture or his race, or of expanding his influence. He is one of the elect who follows the 'Way of God' as if there could be a Way that did not lead to 'God.' We can see all monotheistic religions fighting to impose their god and destroy other gods as if God were not one as they claim. Monotheism is basically the absolute exaltation of the worshiper's own deity over all other aspects of the Divine, all other gods, who must be considered false and dangerous. The very notion of a false god is, however, an obvious fallacy. If there is an all-powerful, all-pervading divinity, how can there be a false god? How can we worship anything that is not Him? Whatever form we try to worship, the worship ultimately goes to Him who is everything.

         "Those who piously worship other gods of whom they are devotees, it is but myself they worship, [though] ignorant of the proper rites." (Bhagavad-gita 9.23.)

         Monotheism thus appears to be the opposite of nondualism, which might as well be called nonmonism, and which leads to the conception of an all-pervading -- that is, from the point of view of our perceptions, an infinitely multiple -- divinity.

Source: http://www.etymonline.com/columns/polytheism.htm