Charan Singh and Kirpal Singh were gurus of Radha Soami which was based in the "Sant Mat" tradition. This is a group of loosely related northern Indian religious paths that began in a medieval revival in opposition to the strict Brahman Vedic religious system characterised by caste, rituals and sacrifices for which dakshina had to be paid. It was also heavily influenced by Islam and its most prestigious and seminal voice, the weaver-poet Kabir is considered by both religions to be one of them. For a reasonable and accurate description see the Wikipedia Sant Mat page.
In it's modern forms Sant Mat religions rely most centrally on the doctrine of bhakti (devotion) to a living human master, the Satguru or Perfect Master, who is considered divine. Both Charan Singh and Kirpal Singh, have taken this doctrine to it's extreme as Andrew Gold explains in "The Lord as Guru: Hindi Sants in North Indian Tradition." This position of a divine guru is obviously a desirable one and modern Sant Mat cults nearly always have considerable dissension and controversy on the succession of the Satguru after the death of the living incumbent. (see "Radhasoami Reality" by Professor Mark Jurgensmeyer, p43). This happened on the death of Sardar Bahadur Jagat Singh in 1951 when Charan Singh became the officially appointed successor and Kirpal Singh became the self-appointed Master claiming that the power to initiate new followers was his.
Radha Soami first became known in the West through "A Search In Secret India" by Paul Brunton in which he discusses (from a superior viewpoint that now sounds Victorian) the Radha Soami colony at Dayalbagh led by His Holiness Sahabji Maharaj Sir Anand Sarup Kt. both a divine guru and a knight of the British Empire. Surely, he was one of a kind. The strand of Radha Soami that has been most influential in the West is that based at Dera Baba Jaimal Singh, located on the banks of the Beas River in the northern Indian state of Punjab, led through the age of the Baby Boomers by Charan Singh.
It became known in the West in "The Path of the Masters" by Julian Johnson published in France (1939) and the USA (1957). It has been revised a few times to remove racist language and politically incorrect ideas. It has been the object of plagiarism and the source of many new bogus American religions. Paul Twitchell's Eckankar (ECK) mail-order religion and John-Rogers Movement For Inner Spiritual Awareness (MSIA) and the various offshoots and spin-offs are based on the direct teachings of Gurinder Singh and plagiarism of Radha Soami books with added esoteric and occult doctrines. Esoteric and occult ideas are de rigeur in American mail-order religions which have a long history going back to the 19th century.
BOOKS BY HUZUR MAHARAJ SAWAN SINGH JI
This is a collection of letters, on matters spiritual, written to the Great Master by his Preceptor and in turn by him to a large number of his disciples and seekers all over the world. The letters are verily a mine of esoteric wealth and Eternal Truths which travellers on the spiritual path cannot but keenly treasure.
Discourses on Sant Mat:
This book, the first of a series, is a rendering into English of the discourses given by the Great Master from time to time. They are based on texts from scriptures dealing with spirituality and are replete with an exposition of the esoteric points involved, the role of ,a living Master and the Audible Life Stream. They delineate a practical way of searching God "within" us and realising Him here and now. The book is a "must" for all seekers after Truth.
Philosophy of the Masters Part I, II and III
Some books are to be read and discarded like the ashes of a cigar. Others are, however, to be thoroughly digested and assimilated. Such books act as a beacon light for the traveller on the path of his moral, social and spiritual life. The Philosophy of the Masters ranks very high amongst this later category of books and contains abundance of material for the guidance of seekers after Truth. It deals with spiritual discipline for ultimate communion with God with scientific precision. This is supported by quotations from Sages, Seers and Mystics of all ages and countries. The whole book will appear in six series of which the present is the first.
Maharaj Sawan Singh the author of this book, assumed the Master's role soon after the death of his own Master in December 1903.
His teachings, popularly known as Sant Mat or the teachings of the Saints, are fundamentally based on connecting the soul currents with the Audible Life Stream - the Word or Shabd. which resounds within us all. This is done by spiritual discipline and by practice of repetition of Holy Names and contemplation, etc., in accordance with the initiation and direction of a living Master.
From 1903 to 1948, the author lived, practised and imparted the above teachings at the Radha Soami Colony at Dera Baba Jaimal Singh, situated near Amritsar - Punjab, India. The institution where he propagated his teachings is known as Radha Soami Satsang Beas. He initiated over a hundred thousand persons into the secrets of the above spiritual practices, which have no connection with religion, caste, creed or nationality. The followers include people from all religions and countries, including the United States of America, South Africa, and the continent of Europe.
There is a considerable number of academic books about the Sant Mat tradition now available.