I have often read that there are some Hindu sects which speak against idol worship. I want to know who they are and what reasons they cite to justify their claims.
The primary sect of Hinduism which rejects idol worship is the reformist sect Arya Samaj, founded by Swami Dayananda Saraswati. Dayananda presents several arguments against idol worship in his book The Satyarth Prakash (The Light of Truth), but here are two of them:
1) He argues that it insults the omnipresence of God to worship him in a localized form, and that any sacrifice or subservience of objects to an idol amounts to denying God's presence in those objects as well (page 242):
Being All-pervading He cannot be imagined to exist in any particular object only. To hold to the contrary would be tantamount to believing that the sovereign Lord of the earth rules over a small cottage to the exclusion of His whole Empire and would be an insult to Him. In like manner, it is a blasphemy against God to imagine Him as existing in one particular object only.
If you believe Him to Omnipresent, why do you pluck flowers fro the garden and offer them to the idol, make a thin paste of Sandal wood and apply this to it, burn incense, beat drums and cymbals, and blow trumpets before it? He pervades your hands, why do you then stand before it with folded palms? He is in your head, why should you then prostrate yourself before the image? He is in food and drinks, why should you then offer them to it? He is in water, why, should you the bathe it? God pervades all these things. What do you worship, the pervader or the pervaded? If the former, why do you then offer flowers, etc., to images made of stone or wood? If the latter, why do you then lay a false claim to the worship of God? Why don't you say that you worship stalks and stones etc., which is the bare truth?
2) He argues that Vedas "positively condemn" idol worship, citing the following quotes (pages 243-244):
(1) "They are enveloped in darkness, in other words, are steeped in ignorance and sunk in the greatest depths of misery who worship the uncreated, eternal prakriti - the material cause of the world - in place of the All-pervading God, but those who worship visible things born of the prakriti, such as the earth, trees, bodies (human and the like) in place of God are enveloped in still greater darkness, in other words, they are extremely foolish, fall into an awful hell of pain and sorrow, and suffer terribly for a long time." YAJUR VEDA 40: 9.
(2)"The formless Supreme Spirit that pervades the universe can have no material representation, likeness or image." YAJUR VEDA 32: 3.
(3) "Do thou offer thy worship to the self-same Brahma (All-pervading God) who transcends the power of speech and is the source thereof by virtue jof His being Omnipotent, and is the support of the Universe. No other Being is worthy of adoration." KENOPANISHAD.
(4)"Do thou offer thy worship to the self-same Brahma who cannot be comprehended by the human mind and yet is cognizant of the inner workings of the mind. Do thou never worship the soul and the intellect in place of God." KENOPANISHAD.
(5) "Do thou offer thy worship to the Being that cannot be seen with eyes, and yet, He it is from whom the power of sight is derived. Do thou never worship the sun, the fire and the lightening and other material objects which are distinct from Him." KENOPANISHAD.
(6) "Do thou offer thy worship to the self-same Brahma who is not the object of auditory perception, and yet He it is who is the bestower of the power of hearing. Do thou never worship sound* etc., in place of the Lord." KENOPANISHAD.
(7) "Do thou offer thy worship to the self-same Brahma who is not influenced by the breathing forces and yet is the director thereof. Do thou never worship the atmosphere in place of the Lord (which is distinct from Him)." KENOPANISHD.
I have read somewhere, 70% of all Hindu scripts is about Atheism.
"Among the various schools of Hindu philosophy, Mimamsa, and Samkhya while not rejecting Brahman, typically rejects a personal God, creator God, or a God with attributes. While Samkhya rejected the idea of an eternal, self-caused, creator God, Mimamsa argued that the Vedas could not have been authored by a deity."
Though this is about not believing in God, it doesn't prohibit worshipping idols.
Brahmo samaj (reform movement within Hinduism) prohibits idol worship