Sacred texts and other main texts: the Upanishad
The Upanishads are the latest texts of the corpus of the Vedas, the textual foundations of the Hindusreligiosity. Vedas (literally [sacred] science) were composed roughly around the 1500–1000 BCE, orally transmitted and put in written form only in recent times (I sec. BCE). This kind of texts are considered not of human creation, and supposed to have been directly revealed to the Rishi (sages, or seers) by the gods themselves. For this reason, their language is considered sacred.
The Upanishads (composed around 500 BCE) contain the earliest emergence of some of the central religious concepts of Hinduism. They are also known as Vedanta ("the end of the Veda"). Such a name does not denote them as mere later texts, but also as the final purpose of the Veda, because they offer the philosophical speculation around the previous Vedic texts. This speculation forged the classic Indian worldview. The Upanishads are considered by Hindus to contain revealed truths (Shruti) concerning the nature of ultimate reality (Brahman) and describing the character of human salvation (Moksha).
More than 200 Upanishads are known, of which the first dozen or so are the oldest and most important and are referred to as the principal or main Upanishads.

Pivotal events for the tradition: The change in the religious worldview
Between the eighth and third centuries BCE multiple factors, such as increasing urbanization, a new social role acquired by women, the emergence of the lay people, brought on a religious renewal. From a previous optimistic view on life, there is a shift towards a pessimistic one: life in this world is conceived as a painful existence, which has to be overcome by renouncing to the social life and carrying on ascetic practices (meditation, feasting) out in the wilderness . This is the worldview of the so called Shramana (renunciants) whose movements challenged the previous orthodoxy of Vedic rituals.
Shramana were against the sacrificial practices connected to the ritual. Instead, they proposed an interiorization of the sacrifice: through the ascetic practice, the ascetic sacrifices him or herself.
The Shramana tradition in part created the concept of the cycle of birth and death (Samsara) and the concept of liberation (Moksha), which became assimilated and became characteristic for Hinduism.
Such situation is reflected in the doctrines of the Upanishad (see above).

Main doctrinal tenets: Common doctrine of Hinduism from Upanishad
What are, then, these doctrine of the Upanishad? They can be explained through four pivotal terms, each of which illuminates the ethical, the eschatological, the soteriological and theological aspects of Hinduism.

Ethics
Karman: originally this word was meant to express the power of the ritual actions to influence reality. Then, with the Upanishad, it became a ethical principle, the universal law of cause and effect , which binds man to his deeds . According to the theory of the remuneration of the acts, the actions that man performs have effect on him in the present and also in the future life. This effect could be positive, negative, or neutral. In each case, it binds man to the cycle of rebirth

Eschatology
(From the Greek eschatos: "last". This word indicates the religious discourses about the end of life, the afterlife and the end of the world).
Samsara is the eternal cycle of rebirth and it is connected to the concept of Karman. The type and quality of rebirth are determined by the actions that are performed in previous life. Based on his Karman, a man could also be reborn in the heavens as a god, or fall into the deepest of hell, or come into new life as a plant or animal. Nonetheless, all these rebirths are temporary, and man is doomed to an endless reincarnation, which is ultimately deemed as suffering. Even if one is born as a god, at the end he will inevitably fall into a lower existence. The rebirth as a man is highly valuated, because it is the only (rare) occasion for attaining liberation (Moskha).
The origin of suffering in Hinduism, that is, the origin of Samsara, is not quite determined: There is not "evil principle" running against a "good principle". What is said to had originate Samsara is an eternal, non-originated, ignorance of man. Ignorance of what? Of the fact that the world is illusory and there's an Absolute behind it (see below).
 

Soteriology
(From the Greek soteria: "salvation". This word indicates the religious discourses concerning the nature of salvation and the ways of obtaining it).
Moksha is liberation from the cycle of reincarnation and it is the ultimate goal of existence. Basically it consists in the stopping of the karmic mechanism. In order to achieve this it is necessary to obtain (or better to experience) the Truth. What truth? That behind the impermanence of Samsara there is an Absolute (see Brhaman below), whose equivalent is the Atman (the "Self"), the sparkle of eternity in every men. Awakened to the truths that reality is just illusion man has no more desire towards the world. He/she is detached from it and therefore he/she does not produce Karman anymore.
Traditionally three ways are envisioned to obtain such  knowledge: ritual, ascetic or devotional, but in the actual practice the three often overlap.

Theology
(From the Greek theos: "god". This word indicates the religious discourses concerning the nature of the Absolute, God, the gods, spirits etc.).
Brahman is the Absolute, which is etymologically linked to the root brh (to expand, to grow). It is the eternal ground of reality, which is behind a world in cyclical flux of birth, development and destruction. Brahman can be conceived either as a impersonal principle or as a supreme god.

Analysis of the sources.
Source n° 1
Brhadaranyaka Upanishad
In this excerpt are found two pivotal aspects of Hindus's thought: the identity between Self (Atman) and Brahman and the role of Karman in the cycle of rebirth.
The Self is basically identified with the intellect, with the senses, but has also a material nature (it is made by the four elements).
Atman, according to its deeds, can become evil and filled with desire, or good and without desire. Some think that Atman can be only made of desire (e.g. of living) but they are wrong. In fact, the man who gets rid of all his desires can finally stop transmigrating, becoming one with Brahman. Whereas the man who desires has attachments towards the world, therefore produces Karman and transmigrates eternally.

Source n° 2
Chandogya Upanishad
This excerpts explain the ethical nature of the law of Karman. Good deeds lead to good rebirth, such as Brahmins (Hindus priest), warrior or merchants (see also p. 8 of this module), whereas evil deeds lead to inferior mode of existence. This excerpt also illustrates the importance of the figure of Brahmins.

Source n° 3
Here is a picture of sadhu, taken near Kedarnath temple in the Indian state of Uttarakhand. His matted hair is one marker of how sadhus reject the settled and conformist values of mainstream Hindu society.

Intercultural & Interdisciplinary information.
Vegetarianism
Given the view of the world as Samsara, the soul of an individual can be reborn in animal or vegetable form.
Moreover, animals are less likely to be killed in order to avoid bad Karman. It follows from this that Hindus tend to show great respect for all kinds of living beings. In fact, the Brahmins, who are responsible for carrying on the various Hindus religious traditions, tend to be vegetarian.

Links to other Modules’ pages
Christianity II. Themes, sec. 3
Judaism II. Themes, sec. 1
Islam II. Themes sec. 2 and 3
Buddhism I. Brief Overview sec. 3 and 4