I consider myself belonging to Shaktism, so I see Shiva as one aspect of Paramātmā. I have seen that unanimously, the Vedas (including Shaiva Upaniṣads) say that Rudra is Brahman. So, Who is this Rudra? How is Rudra evolve to be identified today as Shiva? Is Shiva just an adjective to define Rudra or is it a natural name? Nonshaivites (esp: Vaisnavas) will say Rudra, Maheswara, Vratya etc. is Vishnu. What are some agreed-upon literature that shows the evolution of Rudra is even Lord Shiva?
Kaivalya Upaniṣad says he is Brahma, Śiva, & Viṣṇu. That he has three eyes and a blue (indigo-black) neck/throat.
उमासहायं परमेश्वरं प्रभुं त्रिलोचनं नीलकण्ठं प्रशान्तम् ।
ध्यात्वा मुनिर्गच्छति भूतयोनिं समस्तसाक्षिं तमसः परस्तात् ॥ ७॥
स ब्रह्मा स शिवः सेन्द्रः सोऽक्षरः परमः स्वराट् ।
स एव विष्णुः स प्राणः स कालोऽग्निः स चन्द्रमाः ॥ ८॥
Without a beginning, middle or an end, who is one, all pervading, of blissful consciousness, without form, wonderful, seated with the goddess Uma, the Supreme Lord, the ruler, bearing three eyes, with a blue neck, ever peaceful
—by meditating upon Him the sages attain the source of all creation, the witness of all and that which is beyond all darkness.
He is Brahma, he is Siva, he is Indra. He is the imperishable, supreme self illumined Lord.
He alone is Vishnu. He is the life giving breath. He is time, he is fire, and also the moon.
Sri Rudram Praśna confirms that Rudra also has these features.
नमस्ते अस्तु भगवन्
विश्वेश्वराय महादेवाय त्र्यंबकाय
कालाग्निरुद्राय नीलकण्ठाय म्रुत्युंजयाय
श्रीमन्महादेवाय नमः ॥
namaste astu bhagavan
viśveśvarāya mahādevāya tryaṃbakāya
kālāgnirudrāya nīlakaṇṭhāya mrutyuṃjayāya
Salutations to you God,
Who is the lord of the universe,
Who is the greatest among Gods,
Who has three eyes,
Who destroyed three cities,
Who is the master of the three fires,
Who is the Rudra who burns the world,
Who has a blue neck,
Who won over the God of death,
Who is Lord of every thing,
Who is ever peaceful,
And who is the greatest God with goodness,
The questions are:
who is this Rudra?
Nonshaivites(Vaisnavis) will say Rudra,maheswara,Vratya etc is Vishnu.what are some agreed upon texts that say Rudra is even lord shiva?
I had posted answer to the similar question - Is there any relation or similarity between Rudra & Indra? earlier, as to why Rudra can be viewed as Alimighty/paramatma/BRAHMAN. So I will not repeat the same here.
The Sanskrit word Shiva is an adjective meaning kind, friendly, gracious, or auspicious. In Rig Veda the word Shiva was used as an adjective at places.
For example RV II.20.3 calls Indra as men's auspicious keeper :
स नो युवेन्द्रो जोहूत्रः सखा शिवो नरामस्तु पाता | यः शंसन्तं यः शशमानमूती पचन्तं च सतुवन्तंच परणेषत ||
sa no yuvendro johūtraḥ sakhā śivo narāmastu pātā | yaḥ śaṃsantaṃ yaḥ śaśamānamūtī pacantaṃ ca stuvantaṃca praṇeṣat ||
May Indra, called with solemn invocations. the young, the Friend, be men's auspicious keeper, One who will further with his aid the singer, the toiler, praiser, dresser of oblations.
- In Rudram of Yajurveda, and in subsequent Puranic literature , Rudra was named as Shiva and was described as the SUPREME LORD, who lives on Mount Kailash and whose consort is Parvati, etc.
Coming to the aspect of describing Rudra,maheswara,Vratya etc as Vishnu, by some persons, it is to be understood as projecting their own deity as SUPREME GOD.
In the Veda, the Almighty God is called with different epithets like Indra/Agni/Varuna, etc, which names in subsequent Puranic Literature were converted into demi Gods, and had been given a lower status to Shiva/Vishnu/Shakti, in respective Puranas.
So following of this type of Literature shall be avoided.