In the Nirgun-Sagun state Shiva is worshiped as the Shivalingam.

In the Nirgun-Sagun state Shiva is worshiped as the Shivalingam. According to Linga Purana, the lingam is a complete symbolic representation of the formless Universe Bearer – the oval shaped stone is resembling mark of the Universe and bottom base as the Supreme Power holding the entire Universe in it.

Linga is worshipped because Linga contains everything of this world. In the Linga Purana, once sages have debate regarding why Linga form is worshipped. Goddess Saraswati vist the sages and states ;

"May the questions of the sages stop. The whole world is identical with the Linga. Everything is founded on the Linga. Hence, one shall eschew everything, install the Linga and worship it."

Godesses Saraswati also states that all Gods are present in the Linga form. She states:

"Brahma, Hara, Vishnu, Rama, Dhari, Lakshmi, Dhriti, Smrti, Prajni, Dhara, Durga, Sachi, Rudras, Vasus, Skanda, Visakha, Sakha, Naigamesa, guardians of the quarters, planets, the Ganas, Nandin, Ganapati, Pitras, sages, those lustrous ones beginning with Kubera, Adityas, Vasus, Samkhyaa, the excellent physicians Asvins, Visvedevas, Sadhyas, Pasus, birds: and animals— everything beginning with Brahma and ending with an immobile thing is founded on the Linga. Hence, one shall eschew everything and install the unchanging Linga. If one worships it, it is as good as worshipping everything installed assiduously."

Thus as every God is present in the Linga, it is highly auspicious to worship Linga. Trinities are situated in Linga and it is also stated in the next chapter of Linga Purana:

Lord Brahma resides at the root, Lord Vishnu in the middle. The lord of all unborn Pasupati in the form of Rudra resides at the top.

Similar interpretation is also found in the Skanda Purana ;

“The endless sky (that great void which contains the entire universe) is the Linga, the Earth is its base. At the end of time the entire universe and all the Gods finally merge in the Linga itself.” In yogic lore, the linga is considered the first form to arise when creation occurs, and also the last form before the dissolution of creation. It is therefore seen as an access to Shiva or that which lies beyond physical creation.!! ✨🕉👁🕉✨

Hari Om Tat Sat ~ Hara Hara Mahadev ~ Om Namah Shivaya ✨🌀🕉💙