Tawhid, in Islam, is the absolute and indivisible oneness of God (Allah). It is the foundational principle of the faith, asserting that God is unique, without partners, equals, or rivals. This belief encompasses the affirmation of God's existence, His absolute sovereignty, and His exclusive right to be worshipped.
The Quran emphasizes this in Surah Al-Ikhlas (The Purity): "{Say, 'He is Allah, [who is] One, Allah, the Eternal Refuge. He neither begets nor is begotten, Nor is there to Him any equivalent.}” (Quran 112:1-4). Kurtubî, in his commentary Al-Jami' li-Ahkam al-Qur'an, explains that "Ahad" signifies the One who has no likeness or equal, no consort or child, and no partner. He states that the word "Ahad" is derived from "Wahid" and emphasizes God's uniqueness and His transcendence above any created attributes like begetting or being begotten (Kurtubî — el-Câmi' li-Ahkâmi'l-Kur'ân, s. V20/P243–V20/P250).
Scholars have elaborated on this concept. Imam Maturidi, in his Kitab al-Tawhid, explains that God is one, without a likeness or an equal. He is eternal and self-subsisting, not subject to the limitations of creation, such as being a body or an accident. This uniqueness means He cannot be compared to anything that is created, as anything created is subject to number, change, or potential non-existence (Mâtürîdî — Kitâbü't-Tevhîd, s. V01/P022–V01/P024).
Furthermore, Tawhid means acknowledging that God is the sole sustainer and controller of the universe. He is "As-Samad," the One who is sought after for all needs. As Abd al-Qadir al-Jilani states in his Al-Ghunyah li-Talibi Tariq al-Haqq, God is the One to whom all creatures turn in their necessities. He is the Creator, the Sustainer, and the Ultimate Authority, not needing any support or assistance (Abdülkâdir Geylânî — el-Gunye, s. V01/P118–V01/P122).
Imam Rabbanî, in his Maktubat, further clarifies that the worthiness of worship belongs only to God, who possesses all perfections and is free from all deficiencies. All things are in need of Him, while He is in no need of anything. He is the sole source of benefit and harm, and no one can cause either without His permission. To attribute these qualities to anyone else would imply a deficiency in God, as "the two distinct things are differentiated," and any differentiation would necessitate a lack (İmam Rabbânî — Mektûbât, s. V02/P229–V02/P231).
Tawhid is not merely a verbal affirmation but a deep conviction that permeates one's heart and actions. It means recognizing God's absolute power, His knowledge encompassing all things, and His will being the ultimate determinant. As Imam Ghazali notes in his Ihya Ulum al-Din, the true essence of Tawhid is to see all matters as originating from God, cutting off reliance on intermediaries and worshipping Him alone. He emphasizes that true Tawhid involves seeing all affairs as coming from God, thus worshipping Him alone and not being distracted by intermediaries (Gazâlî — İhyâü Ulûmi'd-Dîn, s. V01/P032–V01/P033).
This belief is the cornerstone of Islam, and understanding its depth is crucial for anyone seeking to comprehend the faith.
This answer has been translated into English. For any doubts, please refer to the original Arabic texts or consult a scholar.