He visited seats of learning in the Malva region and in the holy city of Amritsar in search of a teacher who could instruct him in Sikh philosophy, and ultimately became the pupil of Sant Bishan Singh of village Murala, in Gujrat district (now in Pakistan). Sundar Singh arrived at Murala, on 8 March 1906 and carried on with his study of the Sikh texts till the death, on 28 January 1908, of his mentor. He now launched on his career of expounding the sacred word and holding special gatherings to administer to the seekers the initiatory rites of the Khalsa.To this end, he travelled extensively in the Punjab, especially in the Pothohar, Malva and Lower Chenab Canal Colony (Lyallpur and Sheikhupura districts) areas.
Twice he went on pilgrimage of principal Sikh shrines outside the Punjab such as Nanded and Patna Sahib. During the summer of 1925, he preached in Kashmir valley. Sant Sundar Singh also accepted pupils whom he trained in the interpretation of Scripture. At intervals he took out batches of his students with him on his preaching tours which were devoted primarily to delivering katha, i.e. explanatory discourses on the entire Guru Granth Sahib from beginning to end.
He conducted 19 such courses during his lifetime, besides two similar full scale ones on the Dasam Granth. Instruction was similarly provided in the works of Bhai Gurdas and Bhai Santokh Singh. Sant Sundar Singh also established permanent teaching centres in gurdwaras at several places of which the most important were at his native Bhindar Kalari, at Bopa Rai in Ludhiana district and at Takhtupura, in present day Moga district. For public instruction, he founded Guru Tegh Bahadur school at the village of Rode, near Moga, since upgraded into a college affiliated to Parijab University, Chandigarh.
Besides his world of letters and theology, Sant Sundar Singh involved himself in the wider issues of religious and social reform then engaging the attention of the Sikhs. He took part in the Akali agitation and helped in having the historical shrines at Muktsar and Hehrari brought under the control of the reformists. He visited Nankana Sahib soon after the massacre of the Akalis on 20 February 1921 and participated in Guru ka Bagh morcha (1922) and the morcha at Jaito (1923-25). Sant Giani Sundar Singh was taken ill with dropsy and died at Bopa Rai on 15 February 1930. His work was carried on by his successor, Sant Gurbachan Singh Khalsa.
1. Kartar Singh Khalsa, Khalsa Jivan ate Gurmat Rahit Maryada. Mehta, 1977