A devout Brahmin named Jagannatha lived in Karnataka, around the 12th century. He had a son named Niyamananda. When his upanayana (sacred thread ceremony) was being performed, Narada Muni appeared and taught the Gopal-Mantra to the young boy.
On growing up the boy became a great devotee. He preached the worship of Radha Krishna. Once when a sannyasi arrived at the village, Niyamananda invited him to take food at his house. When the food was ready, night had set in. He went to fetch the sannyasi. But the sannyasi replied that he did not eat after sunset.
Niyamananda was very disappointed. He prayed to Lord Krishna to help him out of his predicament. Lord Krishna called his weapon Chakrayudha, and said that His devotee Niyamananda was unhappy and asked it to stay in the western sky shining like the sun till Niyamananda’s guest ate his food. The Chakrayudha hurried to the place, and shone in the western sky.
Feeling elated at the sudden lifting of the darkness, Niyamananda ran to the sannyasi and appealed to him once more, "Look at the sky, see the sun shining between the branches of the Nimba tree. Please come to my house to eat." The sannyasi went with him and ate the food. Since then Niyamananda has been called Nimbarka, because he showed the sun through the Nimba tree. Arka means the sun.
Sri Nimbarkacharya comes in the line of the Kumara Sampradaya. He was a great ascetic of the Tridandi order. He wrote many books according to the Dvaitavada and he lived mostly near Govardhana at Vrindavana.
Once in a village near the forest of Bilva Paksha, some brahmanas were engaged in the worship of Lord Shiva. They offered him bilva leaves, for a period of one fortnight which satisfied Lord Shiva greatly. Nimbarka was one of these brahmanas. Pleased with Nimbarka's devotion, Lord Shiva appeared before him and told him that at the end of the village was a sacred bael forest where the four Kumaras were absorbed in meditation. By their mercy he would receive transcendental knowledge.
Nimbarka found the four Kumaras shining as brilliantly as the sun. They were seated beneath a tree. They seemed to be no more than five years in age, but they appeared most noble in character as they sat there naked, rapt in meditation. Being excited, Nimbarka cried out, Hare Krishna!
This startled the four brothers. Opening their eyes, they saw the blissful form of an ideal devotee, and they embraced Nimbarka. He fell at their feet and introduced himself. One of the four brothers, Sanat Kumara said, "The Supreme Personality of Godhead, knowing that Kali-yuga will be extremely troublesome for the living entities, has empowered four great souls with devotion and sent them into this world to preach. Ramanuja, Madhva, and Vishnu swami are three and you are the fourth. Lakshmi accepted Ramanuja as a disciple, Brahma accepted Madhva, Rudra accepted Vishnu swami and we have the good fortune to accept you. I have composed a literary work called Sanat Kumara Samhita. After taking initiation from me, you should follow the instructions set forth in that."
The four Kumaras instructed him in the method of worshipping Radha and Krishna with the sentiments of great love called Bhava Marga. In that sacred bael forest Nimbarka began to worship Sri Sri Radha Krishna according to the Sanat Kumara Samhita and he chanted the mantra they had given him. Radha and Krishna revealed Themselves to Nimbarka. They said, "You are very fortunate as you have performed sadhana in Sri Navadvipa. We both combine, and assume one form, as the son of Sachidevi." They then displayed Their form of Gauranga.
Having instructed Nimbarka, Lord Gauranga disappeared. After worshipping the lotus feet of his gurus, Nimbarka left Navadvipa to start preaching.