Mircea Eliade was pretty much the founder of what one would call today the study of comparative religion. His main contribution to the field was to insist on the study of religion as religion - that is, not through the looking glass of sociology, anthropology, history, or any other social science. He was deeply concerned with religion in and of itself, and was interested in knowing what we can learn about it if we accept in on its own terms. Eliade was an Orthodox Christian. He remained one all his life, despite a stint with the Shamans of Yakutsk and at least a two year stint in a cave with a Yogi master. He was also a facist sympathizer, connected to the Legion of the Angel Michael in Romania, who believed that the main result of the study of comparative religion would be to prove that Christianity, and specifically, the narrow brand of Greek Orthodox Christianity practiced in Romania was the most "advanced" religion in the world. Surpising as it may seem, though, this does not destroy the value of his work: in fact most of his scientific works don't even try to prove this point, and essentially, the science he created helped to disprove theories that he held for most of his young life. He eventually became the chairman of the Comparative Religion department of the University of Chicago despite his quasi facist background; Saul Bellow's latest novel, Ravelstein brutally lampoons him as a pompous effete with questionable political leanings, which is probably as true a portrait as the mystic seeker of truth who went looking for enlightenment in the mountains of the himalayas. There are some people that just seemed to be defined by their own internal contradictions.
Eliade wrote four major works in his lifetime, and I give a brief summary of them each.
THE MYTH OF THE ETERNAL RETURN and THE SACRED AND THE PROFANE are what spawned all the Joseph Campbell stuff, and, in an indirect way Star Wars - so you could claim that Luke has Darth Vader on his father's side and effete, facist Romanian intellectual on his mother's side...The main point of the book, which most people today would take for granted, is that religion tends to divide the spheres of life into two categories: the sacred and the profane. The sacred can be definied in large scale terms as any activity, object, period of time, etc., which does not exist for it's own sake, nor for the sake of any material worldly benefit, but rather for the sake of a supernatural, supermaterial power which can be God, enlightenment, or practically anything non-earthly. The final divinity always changes but the dichotomy is always the same. The reason Eliade posited these divisions were made was a deep spiritual need for mankind to feel rooted to a place and time that would be unchangeable, perfect. He noticed that almost all religions spoke about a "perfect" past, a Time of the Ancestors when things were better than they were today; some religions promised a similar state would exist in the future, but other's didn't. All of them, however, promised an Eternal Return - that is, through the management of the sacred man could temporarily return to a period that, despite the use of the word "return" was really outside of time, of space, and idyllic. In general, Eliade noticed that religions held that this space was in the beginning and that the space it referred to was the center. His book is filled with examples of religions which claim to bring the believer back the center of the world at the moment it was created. Being essentially an evolutionist one of his claims for Christianity was that it went past these claims to become a "pure" expression of religion; this idea was rejected by most of his disciples.
Yoga, another of his major books, was probably the founding work of his reputation in Paris (which, in the 30's WAS the intellectual world) and is a remarkable description of Yoga techniques written after Eliade himself traveled to India and found an unknown guru in a Himalayan cave. The study of Yoga dovetailed perfectly with his other works, as Yoga seemed to Eliade to be an attempted to accomplish The Eternal Return through physical exercise, such as breathing techniques and meditation; and his description of Yoga is thorough and contextual.
His fourth great work, which probably should get more attention from psychologists than it does, was Shamanism. This book was developed after a number of field trips and an in-depth analysis of available Russian materials. In the book, Eliade discovers a social process - essentially, among Siberian tribes, young men who are depressed, schizophrenic, or have other significant psychological diseases are actually considered to be budding magicians, suffering because of the internal spirit which wants to be let out and flower. These young men are then apprenticed to the Shaman of the tribe, which does horrible things to them in initiation rituals. Their lips are sown together; they are exposed to the cold and the heat; they are beaten, scoured, and flayed; and in essence, it's a miracle any of them remain alive. Those who survive the initiation, then become "magicians" and their mental illness appears to be channeled - that is, they function relatively normally but at times go into deep trances where they see visions and their own guardian spirit which guides both them and the tribe. The depression, schizophrenia, seemingly disappear. This thesis is VERY controversial (although if you recognized Carlos Castaneda in it you're right - a lot of Castaneda is actually an Eliade ripoff) - there is a lot of argument about whether mental illness actually means the same thing in a small tribe where children are raised by the entire family, and whether or not people with organic mental illnesses are cured through these methods, or included in the process at all.
Other than that, Eliade wrote a History of Religious Ideas which is so dense and dry as to be almost unreadable, and a LOT of fiction, most of which is deservedly unknown outside of Romania. His most controversial life decision was his acceptance of the position of Cultural Attache to Vichy Government from the Facist Romanian Government and his attachment to the legion. His work "Why I believe in the Legion of Saint Michael" is not available in English (at least I've never seen it) but it should be - the question of how an original, brilliant thinker let himself be dirtied through an involvement with anti-semitic thugs continues to haunt Romanian scholars (or at least those Romanian scholars who are not secretly in sympathy with the anti-semitic thugs). The current party line is that he never really was pro-facist, only appeared to be so but make a living. However this seems like a strange insult to make against a man who dedicated one of his books to his spiritual teachers and then wrote "There is a cave in the Himalayas in which I spent years meditating. Now, in the cities of Europe, sometimes the only thing which keeps me sane is the knowledge that, any time I wish, I can go back there..."