Why do we worship God Shiva as Shivaling not as an idol?

 We Hindus worship God Shiva in the form of Shivaling and not as idol of God Shiva . As today is Maha Shivaratri, here is the story regarding this.

Sage Bhrigu once wanted to meet trimurthis. They're God Brahma  , God Shiva and God Vishnu.I talk about only God Shiva . 

When Brighu maharishi has come to Kailasa, divine couple God Shiva and Goddess Parvati were in their private room . So when Sage Bhrigu wanted to enter into the house  , doorkeeper Nandi stopped him saying that divine couple are in their private room .

This made the sage angry thinking that God Shiva is dishonouring his true devotees . In rage   , sage Brighu cursed God Shiva that from now onwards no one will worship him in his true form that is in the form of idol . 

But people will worship him in the form of shapeless stone that is Shivaling . 

From that day onwards    , God Shiva is being worshipped in the form of Shivaling by the people . 

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