What in Shruti (Vedas, Some Upanishads) texts say you must have a guru to be a hindu? Besides, one tells every Hindu has a guru by default : his father. So does it mean that everyone has actually a guru, but that's not necessarily a great one ? Moreover, I found the topic about the initiation of Mantra. The answer is great, that's not the issue. What I am looking for is the Shruti validation, not interpretations.
To finish, how was it in Vedic times for this aspect ?