Srila Prabhupada: "Just like the Vedic religion is called sanatana-dharma because nobody can trace out when this Vedic religion begun. Therefore it is called sanatana-dharma. Every religion in our present experience, it has got a history. Your Christian religion, it has got a history, two thousand years old. Buddhist religion, it has got a history, 2600 years. Muhammadan religion, it has got a history, one thousand years. But if you trace out Vedic religion, you cannot find out the history, date. There is no date. You cannot find out. No historian can give. So therefore it is called sanatana-dharma."
Engagement Lecture -- April 23, 1969, Buffalo