Why do Hindus worship more than one god even though in the Puranas it says brahman is one? Also, it says in the Puranas you shouldn't worship any idols, and yet I see Hindus worshipping idols.
First, Hindus do not worship many gods. gods is a mis-translation of the Sanskrit word deva. A more literal translation of the word deva is 'shining one'. The devas are positions; like governor or chief minister. Ordinary souls or jivas are made devas for a particular cycle of creation. When the cycle ends, the jiva that is that deva has to rebirth again in this world of samsara based upon their previous karma.
Worship of devas is to propitiate or get the help of the deva in some worldly desire or gain. It does help in attaining moksha, or liberation.
Hindus do not worship idols as God, Hindus worship God through the idol. God manifests Himself (or Herself, God has no sex) through the idol.
Swami Vivekananda said at the Parliament of Religions (Complete Works, V1) and here under the heading Addresses at the Parliament of Religions, sub-heading Paper on Hinduism - http://cwsv.belurmath.org/volume_1/vol_1_frame.htm)
As we find that somehow or other, by the laws of our mental constitution, we have to associate our ideas of infinity with the image of the blue sky, or of the sea, so we naturally connect our idea of holiness with the image of a church, a mosque, or a cross. The Hindus have associated the idea of holiness, purity, truth, omnipresence, and such other ideas with different images and forms. But with this difference that while some people devote their whole lives to their idol of a church and never rise higher, because with them religion means an intellectual assent to certain doctrines and doing good to their fellows, the whole religion of the Hindu is centred in realisation. Man is to become divine by realising the divine. Idols or temples or churches or books are only the supports, the helps, of his spiritual childhood: but on and on he must progress.
He must not stop anywhere. "External worship, material worship," say the scriptures, "is the lowest stage; struggling to rise high, mental prayer is the next stage, but the highest stage is when the Lord has been realised." Mark, the same earnest man who is kneeling before the idol tells you, "Him the Sun cannot express, nor the moon, nor the stars, the lightning cannot express Him, nor what we speak of as fire; through Him they shine." But he does not abuse any one's idol or call its worship sin. He recognises in it a necessary stage of life. "The child is father of the man." Would it be right for an old man to say that childhood is a sin or youth a sin?
If a man can realise his divine nature with the help of an image, would it be right to call that a sin? Nor even when he has passed that stage, should he call it an error. To the Hindu, man is not travelling from error to truth, but from truth to truth, from lower to higher truth. To him all the religions, from the lowest fetishism to the highest absolutism, mean so many attempts of the human soul to grasp and realise the Infinite, each determined by the conditions of its birth and association, and each of these marks a stage of progress; and every soul is a young eagle soaring higher and higher, gathering more and more strength, till it reaches the Glorious Sun.
Unity in variety is the plan of nature, and the Hindu has recognised it. Every other religion lays down certain fixed dogmas, and tries to force society to adopt them. It places before society only one coat which must fit Jack and John and Henry, all alike. If it does not fit John or Henry, he must go without a coat to cover his body. The Hindus have discovered that the absolute can only be realised, or thought of, or stated, through the relative, and the images, crosses, and crescents are simply so many symbols — so many pegs to hang the spiritual ideas on. It is not that this help is necessary for every one, but those that do not need it have no right to say that it is wrong. Nor is it compulsory in Hinduism.
One thing I must tell you. Idolatry in India does not mean anything horrible. It is not the mother of harlots. On the other hand, it is the attempt of undeveloped minds to grasp high spiritual truths. The Hindus have their faults, they sometimes have their exceptions; but mark this, they are always for punishing their own bodies, and never for cutting the throats of their neighbours. If the Hindu fanatic burns himself on the pyre, he never lights the fire of Inquisition. And even this cannot be laid at the door of his religion any more than the burning of witches can be laid at the door of Christianity.
To the Hindu, then, the whole world of religions is only a travelling, a coming up, of different men and women, through various conditions and circumstances, to the same goal. Every religion is only evolving a God out of the material man, and the same God is the inspirer of all of them. Why, then, are there so many contradictions? They are only apparent, says the Hindu. The contradictions come from the same truth adapting itself to the varying circumstances of different natures.
It is the same light coming through glasses of different colours. And these little variations are necessary for purposes of adaptation. But in the heart of everything the same truth reigns. The Lord has declared to the Hindu in His incarnation as Krishna, "I am in every religion as the thread through a string of pearls. Wherever thou seest extraordinary holiness and extraordinary power raising and purifying humanity, know thou that I am there." And what has been the result? I challenge the world to find, throughout the whole system of Sanskrit philosophy, any such expression as that the Hindu alone will be saved and not others. Says Vyasa, "We find perfect men even beyond the pale of our caste and creed." One thing more. How, then, can the Hindu, whose whole fabric of thought centres in God, believe in Buddhism which is agnostic, or in Jainism which is atheistic?
The Buddhists or the Jains do not depend upon God; but the whole force of their religion is directed to the great central truth in every religion, to evolve a God out of man. They have not seen the Father, but they have seen the Son. And he that hath seen the Son hath seen the Father also.
This, brethren, is a short sketch of the religious ideas of the Hindus. The Hindu may have failed to carry out all his plans, but if there is ever to be a universal religion, it must be one which will have no location in place or time; which will be infinite like the God it will preach, and whose sun will shine upon the followers of Krishna and of Christ, on saints and sinners alike; which will not be Brahminic or Buddhistic, Christian or Mohammedan, but the sum total of all these, and still have infinite space for development; which in its catholicity will embrace in its infinite arms, and find a place for, every human being, from the lowest grovelling savage not far removed from the brute, to the highest man towering by the virtues of his head and heart almost above humanity, making society stand in awe of him and doubt his human nature. It will be a religion which will have no place for persecution or intolerance in its polity, which will recognise divinity in every man and woman, and whose whole scope, whose whole force, will be created in aiding humanity to realise its own true, divine nature.
Ekam Sat Vipra Bahudha Vadanti (Rig Veda I.164.46) - Truth is One, men call it by various names.