What are the fundamentals and principles of religion, and do they differ?

The fundamentals of religion are matters that must be believed, along with the proofs for these matters. They include Tawhid (Oneness of God), the Attributes [of God], Divine Decree (Qadar), Prophethood, and the Hereafter. Allah and His Messenger have expounded upon these matters with a complete explanation. The Salaf (pious predecessors) believed that Allah clarified the necessary rational proofs for knowing these matters in the Quran and Sunnah, unlike the Mutakallimun (speculative theologians) and philosophers who hold that the indication of religious texts depends on the truthfulness of the one conveying the information. The fundamentals of religion (Usul al-Din) and its foundational principles (Qawa'iduhu) are one and the same in meaning, and they encompass the rights of Allah and His servants, as well as the interests of this world and the Hereafter.