The best response that summarizes the fundamental beliefs of Islam and its religious practices based on the provided primary source is:
"The Quran is a sacred scripture and teachings that all Muslims follow. The primary-source scripture is the first pillar: profession of faith."
This response highlights the importance of the Quran as the sacred text and identifies the declaration of faith, or Shahada, as the first of the Five Pillars of Islam, which symbolizes the fundamental belief in Islam.