Chapter Six Defining the Term: "Usul al-Din" Know, my dear brother, that the term "Usul al-Din" (the fundamentals of religion) must be defined in a way that conforms to Islamic law. Otherwise, you will fall into the error of declaring Muslims to be infidels without any legal proof. This is because the extremists, by not properly defining this term, end up declaring as infidels "anyone who is ignorant of the Islamic ruling regarding the disbelief of one who commits an act of disbelief and attributes himself to Islam," claiming that declaring someone an infidel is part of the fundamentals of religion, and that whoever does not declare this person an infidel has failed in the fundamentals of religion and has not truly embraced Islam.

To clarify the fallacy of this statement, we must explain the concept of "Usul al-Din." We say: The term "Usul al-Din" refers to one of two things: First: The general fundamental principle of the religion of Islam. This is the religion with which Allah sent all the messengers, for which Allah sent down the books, and upon which He created people. It is for this reason that Allah created creation. Its fundamental principle is the oneness of Allah and dedicating worship to Him alone.

Allah the Almighty says: "And We did not send before you any messenger except that We revealed to him, 'There is no deity except Me, so worship Me.'" [Surah Al-Nahl: 36].

And He the Almighty says: "And We did not send before you any messenger except that We revealed to him, 'There is no deity except Me, so worship Me.'" [Surah Al-Anbiya: 25].