Q 14: What is the proof that the word "Islam" is used to denote the whole matter of religion?

A--- Allah the Most High says: {Truly, the religion with Allah is Islam}. (Al' Imran, 19)

Prophet Muhammad (Peace be upon him) says: "Islam started in loneliness and will end as it started in loneliness, blessed be the lonely".

(Sahih Muslim)

He (Peace be upon him) says: "The best of deeds in Islam is Belief in Allah". (Musnad Ahmed)

Reference: 200 FAQ On Muslims Belief - Shiekh Hafiz Ibn Ahmed Al Hakami