The Chandogya Upanisad is found in the Chandogya Brahmana of the Sama Veda. The Chandogya Brahmana has ten chapters altogether, the last eight of which constitute the Upanisad. The name of this Upanisad is derived from the word chandas. A person who sings the Sama Veda (chandas) is called a Chandoga. And the beliefs and practices of the Chandogas are set forth in the Chandogya Upanisad.
Each of the chapters of the Chandogya Upanisad is important. The first five chapters are about worship and meditation (that is, they are dualistic); whereas from the sixth chapter on, they are about Brahman (and in that sense they are more or less non-dualistic). The purpose of the meditation chapters is to emphasize the need for a pure heart. Only when the heart is pure can instructions about Brahman be effective.
The Chandogya Upanisad occupies a high place among the extant Upanisads. Its language is simple and it tells many stories, but its subject-matter is profound. It takes care of the needs of ordinary people as well as those who are highly intellectual. It is like a loving mother, leading her child by the hand.
The Upanisad begins by telling people what they have to do to maintain spiritual progress, and it offers two options: either ritualistic worship as prescribed by the scriptures, or meditation on the sublime Brahman, again as prescribed by the scriptures. The Chandogya concedes that for most people the first option is the best. They are people incapable of thinking of anything beyond the reach of sense perception. Brahman, beyond thought and speech, means nothing to them. They have many desires and they would be happy to have those desires fulfilled. Ritualistic worship offers them the opportunity to attain just those things they desire.
But the Upanisad makes no secret of the fact that everything that ritualistic worship offers-money, health, beauty, power, even heaven-is short-lived. If a person wants permanent peace and happiness, he has to have liberation-liberation from the bondage of desires. And this is attainable only through Self-knowledge. As the Svetasvatara Upanisad (III. 8) says: 'Tameva viditva' Timrtyumeti Nanyah Pantha vidyate yanaya - if you know that [Brahman] you overcome death. There is no other way [for liberation].' This is a warning to those who engage in ritualistic worship, thinking they will get everything they want through it. The Svetasvatara Upanisad says that the only way to attain permanent peace and happiness is through knowledge of Brahman-that is, through Self-knowledge.
The Chandogya Upanisad sounds the same warning, and having done this, it tries to draw our attention to Self-knowledge. As a first step in this direction, it gives an instruction on the udgitha, the recitation of Om. This is part of the worship a person has to perform to bring the mind under control. Though the goal is Self-knowledge, a person must first control the mind by doing some upasana (spiritual practices). The Isa Upanisad (verse 11) also gives this advice. It asks a person to combine vidya (worship of gods and goddesses) and avidya (performing sacrifices.). This is the path of gradual liberation.
Sarikara also recommends this for those who are not yet ready for the more difficult path of renunciation. In his commentary on the Chandogya Upanisad, he advises that they practice the udgltha, the recitation of Om. A sure way of controlling the mind is to recite Om whenever you do or say anything. To emphasize its importance, the Chandogya Upanisad devotes its first five chapters to the udgltha. From then on it discusses nothing but Brahman.
The Chandogya Upanisad was often quoted by Sankara in his commentaries to establish his philosophy of non-dualism. In fact, there is hardly any issue in Vedanta which is not discussed in the Chandogya. It is a complete manual on Vedanta.