We shall make an attempt to do the impossible: to give an overview of guruism, which has in the last 100 years been and still is influential in Europe.
In fact the expansion of gurus came as a result of Hindu nationalism already from the 1820ies. We bypass however that part of the Hindu mission, for its influence was minor outside India. In India we refer to such movements as Brahmo Samaj and Arya Samaj, two very different types of renewal, the first modernist, the second traditionalist, but both heterodox in relation to the Sanathana Dharma, i.e. the traditional Hinduism.
The full defense of Hindu religion, however, came into being by Ramakrishna and Theosophy. Ramakrishna himself is in the background, but his first disciple Vivekananda created the Ramakrishna mission and made it world known in Chicago when the world conference of religionists ran their "parliament", i.e. convention, in 1893. Vivekananda was the inspiration with his self-created image of an Eastern mystic. At the same time and at the same place Annie Besant propagated theosophy as another option for those who looked for the light from the East. She had succeeded Madame Blavatsky as the leader for theosophists worldwide, since "the founding mother" had fallen from grace by swindling in order to promote faithful theosophical belief.
Theosophy is still there, but subdivided in many sectarian forms of theosophical spiritualism. Each area has a version of theosophy.
Theosophy often mixes with different orders of occult nature, such as Ordo Templi Orientis (OTO). Aleister Crowley is the "great beast", as he called himself, and his influence is found all over the place in "magick" and tantric yoga. Occultism became very popular already from the beginning of the dark 20th century. Refugees from Russia such as Ouspensky and Gurdjieff added their weird influence to the choir.
The renewal of Hinduism took place in many different forms, but all more or less nationalist. They all worshiped first of all "Mother India", who often for them was identified with "Mother Kali" or which name they preferred for the Mother Goddess, Mother Kundalini.
This renewal first of all meant a confiscation of Yoga and the Yogic ideology, which willingly became nationalized. Thereby the most weird parts of the Tantric tradition had to be kept secret. Let me illustrate this by an anecdote from my own experience in the US.
I visited in St. Louis the first and decisive Vedantic Center in the West ten years ago and experienced what I call split level religion. The local swami was a sophisticated monk, who took us around in the center. He pointed to the emblem of Vivekananda, which however was radically modified, since the serpent power was omitted. His comment was: We could not tell the North-Americans about the serpent power, could we? But upstairs in the chapel you will find the Mother Kundalini!
When he took the visitors into the library, he showed them the usual three photos of "the founding fathers", and there were in fact three men, and no women. No founding mother, as normal according to tradition. Again the modern swami became defensive and said: We could not tell the North American, that a swami (Vivekananda) had a consort (the so-called Mother) could we, but upstairs in the library you will find Holy Mother. She had on the wall been substituted by the first president of the Ramakrishna mission.
Religion by split level tells a very important story. It is the model for the Hindu missionary outreach. They simply tell another story, fitted for modern Western people. No Kundalini, no sex, no yoga in the original meaning of yoga.
But still the Hindu mission is at the bottom. Neo-Hindus still remain Hindus. The transformation can even be illustrated in a more radical way, for it also includes the soul of Hinduism, the belief system around samsara or reincarnation or rebirth.
The heart of both Hinduism and Buddhism is belief in reincarnation. From that belief all yoga is derived and without that belief yoga is meaningless.
Classical Hinduism considers reincarnation the same as perdition. Salvation is the opposite. You are saved from rebirth, the eternal circle of reincarnation. Yoga is the technology for that salvation from rebirth.
Therefore the key word in yogic terminology is stoppage or harnessing. You have to stop all life functions and return to a life, which has no signs of life and in fact is death. But not just death as the end of this life, but death as the end of all life, the eternal death. The goal is to become "a living dead", and thereby do away with Samsara.
This has been the key to all genuine Hindu schools and in fact also to the main core of Buddhist schools. Life eternal is for them endless life as suffering and that is endless suffering. In order to put an end to suffering, you have to put an end to life, but not just to this life, but to the life-process as such.
It is in fact a giant goal. "While you live you must die, in order to die when you die..." This formula is a statement from a prominent Hindu yogi, who is in fact also a Buddhist master.
The Christian counterpart is formulated like this on the walls in the Schleswig cathedral: "While you live you must die in order not to die when you die." This is about eternal life as salvation, but the yogic formula deals with the possibility of eternal death, the total stoppage as the only way out of suffering and life.
The transformation of the classical rebirth-theory took place, strangely enough, by an amalgamation with evolutionary theosophy. Annie Besant more than any other has described this fusion, whereby evolution became the backbone of a new sort of reincarnation.
That all happened as a part of the general Westernization of India. After the first attraction of the Christian missionaries which to some part created a Christianization, the majority of the attracted Indians decided to go for a general Modernization in the Westernized form of Christian culture. Not conversion to Christianity but modernization and transformation to some westernized lifestyle became the option.
Thereby the rejection of life and the Hindu ascetical pretensions were no longer of any general meaning. Some of course went on as sannyasins and monks, but that was not any longer the real future of the young men. They wanted the modern life with all or most of its possibilities.
What then with "the eternal way of life", the "Sanathana Dharma"? The real answer to that was given by Neo-Hinduism, who found out how to eat the cake and keep it. The modernizing youth did not reject Hinduism, but transformed it and modernized it.
Karma now became the formula for action for your country. Sometimes it was even interpreted as service for your neighbor. Samsara first of all was reinterpreted as rebirth into a new life again and again, not necessarily after death, but also after death. New life became an attractive possibility - another chance, so to speak. One life is not enough, you will get more possibilities, depending on your own effort and the benevolence of the gods.
In many ways this new religion is both Hindu and Christian, if one does not ask for too much. It is colored by both religions, but belong to neither. It is however, recognized as Hinduism by the modern gurus who have made this new religion the great possibility for themselves to promote what they call "the old way" but which is definitely "a new way."Evolutionary thought was thus fused with reincarnational beliefs, and evolutionary reincarnation became the ideology. That created a general New Age religiosity, which soon became a sort of universal spirituality which attracts millions of people worldwide.