Do the Vedas prohibit idol worship?
The verse you are referring to is as below:
andhaṁ tamaḥ praviśanti ye 'sambhūtim upāsate
tato bhūya iva te tamo ya u sambhūtyām ratāḥ
This is present in the Isha Upanishad or the Shukla Yajur Veda chapter forty. But to say that this verse prohibits idol worship would be incorrect. All that it says is, those who worship only the asambhuta (which has not originated) and those who worship only the sambhuta (which has originated) enter into darkness.
But the thing is, because Sanskrit words can mean multiple things, people give it different meanings. But simply speaking, asambhuta here means the unmanifested absolute formless mode of supreme Brahman and sambhuta means the different manifested forms like the devas or demigods. It is because, the absolute neither comes to exist nor ceases to exist (without origin), but the various gods come to exist and also after their time is over cease to exist (with origin). Similarly, there is another verse which forbids both knowledge and ignorance:
andhaṁ tamaḥ praviśanti ye 'vidyām upāsate
tato bhūya iva te tamo ya u vidyāyām ratāḥ [Isha Up. - 9]
Now tell me, what kind of logic is this! It is understandable if we say one will enter darkness if he worships or follows ignorance, but why would any one enter darkness if he follows knowledge?
So these verses only encourage to have a complete knowledge of the both the aspects instead of following or knowing only one partially. It is because both knowledge and ignorance are part of God:
vidyāvidye mama tanū [SB - 11.11.3]
- Both knowledge and ignorance are my body (energy potencies)
And God is both with and without forms:
dve vāva brahmaṇo rūpe, mūrtaṃ caivāmūrtaṃ ca [Brh. Up - 2.3.1]
- God (Brahman) has two modes, formless (nirakara, asambhuta) as well as form (sakar, sambhuta).
The Vedas and scriptures are full of contradicting statements for a certain reason. If one takes up only one statement and tries to define everything else as per it, then he will only reach biased and wrong conclusions. Complete knowledge is always required. So another verse of that same Upanishad explicitly mentions to known both knowledge and ignorance. Because only by knowing both the modes of God that one will be able to have the complete and absolute knowledge:
vidyāṁ cāvidyāṁ ca yas tad vedobhayaṁ saha
avidyayā mṛtyuṁ tīrtvā vidyayāmṛtam aśnute [Isha Up. - 11]
Only one who can learn the process of nescience (avidya) and that of transcendental knowledge (vidya) side by side can transcend the influence of repeated birth and death and enjoy the full blessings of immortality.
So don't take any such arguments seriously. Idol worship is neither mandatory nor prohibited in Hinduism. In fact puranas like Shrimad Bhagavatam itself mention the process of deity form worship of the God and what the idols can be made of:
śailī dāru-mayī lauhī lepyā lekhyā ca saikatī
mano-mayī maṇi-mayī pratimāṣṭa-vidhā smṛtā [SB - 11.27.12]
The Deity form of the Lord is said to appear in eight varieties — stone, wood, metal, earth, paint, sand, the mind or jewels.
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