Adherents hold that Hinduism—one of the principal faiths in the modern world, with about one billion followers—is the world’s oldest religion, with complete scriptural texts dating back 3,000 years. The oral tradition that gave rise to the Mahabharata, for example, probably dates to about 850 BCE, although its written Sanskrit form is about 400 years younger.
Zoroastrianism, the chief pre-Islamic religion practiced in Iran, draws on some of those Sanskrit oral compositions and, later, written texts. Its founder, Zarathushtra, wrote down hymns that predate written Sanskrit literature, which makes it possible to claim that Zoroastrianism is older than Hinduism, formally codified.
Judaism dates to great antiquity as well, with an oral tradition that is nearly 4,000 years old and written texts that may be older than the Sanskrit and Avestan texts of Hinduism and Zoroastrianism. For example, the Pentateuch (the first five books of the Bible) has some written elements that are thought to have originated in the mid-2nd millennium BCE.
The ultimate answer to the question depends in good part on what is meant by the term religion and its evolution: Does it require written texts? Can those texts be precisely dated? Must it be the same now as it was at its founding? For example, Judaism, Hinduism, and Zoroastrianism as they are practiced today have taken certain departures from their most ancient forms, as have newer world religions such as Christianity and Islam. Suffice it to say that most of the world religions that we know today have roots in practices that are thousands of years old.