|All the four Kumaras|
|Affiliation||Rishi, Avatar of Vishnu|
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Sage Sanatkumara was one of the Four Kumaras, the four Manasputras (mind-born-sons) or spiritual sons of Brahma according to Puranic texts of Hinduism, including the Bhagavata Purana, whose other sons were Sanaka, Sanatana, and Sanandana. Sanatkumara in Sanskrit means "eternal youth". Though in Mahabharata, total seven sons are mentioned, namely: Aniruddha, Sana, Sanatsujata, Sanaka, Sanandana, Sanatkumara, Kapila, and Sanatana and further mentions that,"Knowledge comes to these seven rishis, of itself (without being dependent on study or exertion). These seven are wedded to the religion of Nivritti (inward contemplation). They are the foremost of all persons conversant with Yoga. They are possessed also of deep knowledge of the Sankhya philosophy. They are preceptors of the scriptures on duty and it is they that introduce the duties of the religion of Nivritti (inward contemplation), and cause them to flow in the worlds".
The Chandogya Upanishad, Chapter seven, is about Sanatkumara's Instructions on Bhuma-Vidya to celestial sage Narada, Sanatkumara finds mention across Mahabharta, as a great sage, who dispels doubts  and the preceptor in all matters affecting Yoga 
Sanatkumara as Lord Karthikeya
Boudhayana Kalpa Sutra and Taittiriya Upanishad states that Sage Sanatkumara appeared as Lord Subrahmanya, also known as Lord Muruga in south, due to the request of Lord Shiva and Goddess Parvati. Lord Muruga is one of six main Deities of Hindu.
Legend of Jaya and Vijaya
Bhagavata Purana mentions the story of the Four Kumaras, visiting Vaikuntha, the abode of Vishnu, where his gate keepers, Jaya and Vijaya, stop them, taking them to be children, as due to their 'tapas', the Four Kumaras, looked like children or eternally young; upon which the four Kumaras cursed them to take birth on earth, as human beings.
As the legend continues, Vishnu intervened and offered them the option of taking three births as his enemy, whereupon they would be restored to their former position.
Subsequently, in Satya Yuga, they were born as Hiranyakashipu and Hiranyaksha, to Diti (daughter of Daksha) and Kasyapa. Lord Vishnu undertook the Varaha Avatar to kill Hiranyaksha, and the Narasimha avatar to kill Hiranyakasipu.
In the second life, during the Treta Yuga, they were born as Ravana and Kumbhakarna and defeated by Rama avatar as mentioned in the epic Ramayana. Finally, in their third and final life in Dwapara Yuga, they were born as Sishupala and Dantavakra during the time of Krishna avatar, also mentioned in epic Mahabharatha.
- Bhag-P 3.15.12 Bhagvata Purana "Lord Brahma said: My four sons Sanaka, Sanatana, Sanandana and Sanatkumara, who were born from my mind"
- SAGE SANATKUMARA
- "The Vedic dharma (religion) is verily twofold, characterised by Pravritti (social action) and Nivritti (inward contemplation), designed to promote order in the world; this twofold dharma has in view the true social welfare and spiritual emancipation of all beings." -Adi Shankaracharaya (A.D. 788-820).
- Vaisampayana continued... The Mahabharata translated by Kisari Mohan Ganguli (1883 -1896), Book 12: Santi Parva, Part 3, Section: CCCXLI. p. 147 "The puissant Lord who is charged with the creation of all the worlds is called Aniruddha, Sana, Sanatsujata, Sanaka, Sanandana, Sanatkumara, Kapila, and Sanatana numbering the seventh,--these seven Rishis are known as the spiritual sons of Brahman. Their knowledge comes to them of itself (without being dependent on study or exertion). These seven are wedded to the religion of Nivritti. They are the foremost of all persons conversant with Yoga. They are possessed also of deep knowledge of the Sankhya philosophy. They are preceptors of the scriptures on duty and it is they that introduce the duties of the religion of Nivritti, and cause them to flow in the worlds.
- Part Seven Chapter I — Dialogue between Narada and Sanatkumara by Swami Nikhilananda.
- Chapter Three: Sanatkumara's Instructions on Bhuma-Vidya by Swami Krishnananda, The Divine Life Society, Shivananda Ashram, Rishikesh, India.
- Markandeya continued.. The Mahabharata translated by Kisari Mohan Ganguli (1883 -1896), Book 3: Vana Parva: Markandeya-Samasya Parva, Section: CLXXXIV. p. 372 "Markandeya continued, 'On hearing this, the great-mind Munis went instantly to Sanatkumara who was well versed in religion to clear their doubt...
- Sanatkumara The Mahabharata translated by Kisari Mohan Ganguli (1883 -1896), Book 12: Santi Parva: Mokshadharma Parva: Section:CCLXXX. p. 295."While they were thus conversing with each other there came unto them the great sage Sanatkumara of righteous soul for the purpose of dispelling their doubts. Worshipped by the prince of Asuras and by the sage Usanas, that foremost of sages sat down on a costly seat.".
- Narada The Mahabharata translated by Kisari Mohan Ganguli (1883 -1896), Book 2: Sabha Parva: Lokapala Sabhakhayana Parva, section:XI. p. 25 And Daksha, Prachetas, Pulaha, Marichi, the master Kasyapa, Bhrigu, Atri, and Vasistha and Gautama, and also Angiras, and Pulastya, Kraut, Prahlada, and Kardama, these Prajapatis, and Angirasa of the Atharvan Veda, the Valikhilyas, the Marichipas; Intelligence, Space, Knowledge, Air, Heat, Water, Earth, Sound, Touch, Form, Taste, Scent; Nature, and the Modes (of Nature), and the elemental and prime causes of the world,--all stay in that mansion beside the lord Brahma. And Agastya of great energy, and Markandeya, of great ascetic power, and Jamadagni and Bharadwaja, and Samvarta, and Chyavana, and exalted Durvasa, and the virtuous Rishyasringa, the illustrious Sanatkumara of great ascetic merit and the preceptor in all matters affecting Yoga..."
- Kanakhala The Mahabharata translated by Kisari Mohan Ganguli (1883 -1896), Book 3: Vana Parva: Tirtha-yatra Parva: Section CXXXV. "Here, O king, before thee is the Kanakhala range, the favourite resort of sages. 'And yonder is the mighty river Ganga. Here, in ancient times, the holy sage Sanatkumara attained ascetic success. O scion of the Ajamidha race, by performing thy ablutions here in this river, thou wilt be freed from all thy sins."