How does the Qur’an being universal makes it the only authority Muslims need?

Some Muslims may consider the Qur’an to be the only authority a Muslim needs because it is universal. The word of Allah does not apply to any specific religion, unlike the other holy books, so the messages in the Qur’an can sustain validity in all conditions. The Quran quote, “And we have not sent you but onto the whole of mankind, a bearer of glad tidings”, which suggests the Qur’an is for all people and revealed to Muhammed so that he may enlighten all with it.