We, Hindus, believe in one god. Nevertheless we do worship many deities. Ostensibly, this is a paradox. Many people think that Hinduism is a polytheistic religion. But, to put it in proper perspective, the Hindu faith is monotheistic. Let me explain.
Hindu scriptures, Upanishads, call God as Brahman (meaning infinite expanse). The composers of Upanishads were ancient Vedic seers who attained a high spiritual level through meditation. These exalted seers concentrated their thoughts on God and perceived Brahman as one without a second, an impersonal, formless, infinite, eternal, absolute and unchanging reality. Brahman is transcendent as well as immanent.
The Vedic sages in their great wisdom realized that for the common people, the concept of formless, impersonal, omnipotent and omnipresent God would be difficult if not impossible to comprehend. Forms were given to the formless, so that the ordinary people could relate to God and show their love and devotion for him.
The sages in their meditation used to perceive different divine manifestations and described them in verses, which eventually became the prayer verses for these deities. Hindus worship many different forms as manifestations of divinity. Many of these images are anthropomorphic.
During services the worshippers connect to God through these images and pour out their devotion through prayers, rituals and chanting mantras from holy books.
The Hindu deities are not idols; they are religious icons.