For more a thousand years, the Surangama (Shurangama) Sutra — the "Sutra of the Indestructible” has been held in great esteem in the Mahāyāna Buddhist countries of East and Southeast Asia. Regarded as a complete and practical manual that leads to enlightenment, it gives instruction in the understanding of the Buddha-nature, the potential within all beings for becoming a Buddha.In much of the Sutra the Buddha Sakyamuni instructs the young monk Ānanda, teaching him to turn the attention of his sense-faculties inward in order to achieve a focused state of meditation known as samādhi. By practicing the Surangama Samādhi, anyone who maintains purity of conduct and develops right understanding can gain awakening. At the heart of the Sutra is the Surangama Mantra. The practice of this mantra can eliminate internal or external obstacles that lie in the way of spiritual progress. Due to its wealth of theoretical and practical instruction the Surangama Sutra has often been the first major text to be studied by newly ordained monks in China, particularly those of the Chan School.
Not to be confused with the Surangama Samadhi Sutra, the Surangama Sutra is an apocryphal text of Chinese origin dating back to the eighth century AD. Its greatest flaw is that it attempts to philosophically (i.e., intellectually) investigate the perception of reality - an approach antithetical to Buddhism itself - inevitably falling victim to the naiveties of the era in which it was written.
Thus, interpretations of reality are given that may seem amusing, viewing matter as composed of primordial elements (earth, air, water, fire) that hark back to the philosophy of Empedocles (5th century BC), or discussing reincarnations made of spirits that leave bodies at their death to follow 'lights' that lead them to inhabit 'new bodies' and other unscientific notions.
Perhaps its greatest merit is to show the grandeur of Buddhism by antithesis, committing all the errors from which the great masters taught to keep a distance, namely not relying on reasoning to arrive at an understanding of reality.
Reading it is laborious, repetitive, but above all superfluous unless one has historical interests.