One reason why holy books are still important to Muslims in the modern world is because they are the only source for Muslims to be the word of Allah. Muhammad was revealed “the Quran from the One all wise and knowing (Quran 27:6). and is the only book believed to be the direct word of Allah. This is important because it provides a direct insight to what Allah wants for humans to go to Jannah, which the Hadith may be less capable of due to the fact that it is the Prophet Muhammad’s teachings (who isn’t infallible like Allah)
