Consider Ayat 65 of Surat Nisa'i in the Qur'an.

As-Salaam alaikum,
In Ayat 65 of Surat Nisa'i of the Holy Qur'an (4:65), Allah Subhanahu Wa Ta'ala, says:--

   "BUT NO! BY YOUR LORD! THEY WILL NEVER TRULY BELIEVE UNTIL THEY MAKE
    YOU (O Muhammad) THE JUDGE OF WHAT IS IN DISPUTE BETWEEN THEM AND
    FIND WITHIN THEMSELVES NO DISLIKE OF THAT WHICH YOU DECIDE, AND
    THEY SUBMIT WITH FULL SUBMISSION."

Thus Allah Ta'ala is saying that this affair is not like they assume that they can connect to Allah without the necessity of following His Messenger, Sallallahu alaihi Wasallam. "THEY WILL NEVER TRULY BELIEVE", in Me and in you O Muhammad, "UNTIL THEY MAKE YOU THE JUDGE", ie make you the criterion of judgement regarding their rights; "..AND FIND WITHIN THEMSELVES NO DISLIKE OF THAT WHICH YOU DECIDE", ie they find no constraints or distress in what you have judged in your judgement; "AND THEY SUBMIT WITH FULL SUBMISSION"-- ie that they confine themselves completely and perfectly to all your judgements thoroughly, and be guided in all they do by your decisions, as a covenant between you and them.

 This Noble Ayat is an answer to those who claim that it is sufficient for them in their religion to follow the Book of Allah only, without following the Sunnah of His Messenger. They claim that the Sunnah and the Prophetic Traditions have weaknesses and obscurities. Whoever claims thus faces the danger of disbelief (Kufr) as this Noble Ayat clarifies. Beware!!