Explanation of the Four Principles by Shaykh Saleh ibn Fawzan
Allah the Most High says, "I created not the Jinn and humans except that they should worship me (alone)."
Any type of worship mingles with even a mustard seed amount of shirk, all of the worship unto Allah (All kinds of Ibadah) will be destroyed and invalidated. And whoever committed Shirk would be thrown ito the Hell-Fire eternally. So every muslim should realize all the four principles of Shirk are the most important matter for him to learn.