Tawhid — the oneness of God
Tawhid is the heart and foundation of all of Islam: the belief that God (Allah) is one, unique and without equal, having no partners, parents, children, or likeness, and that He alone is worthy of worship.
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This pure oneness is expressed concisely in the short surah 112 (Al-Ikhlas). Scholars distinguish several of its aspects: oneness in lordship (He alone creates and governs), oneness in worship (to turn and pray to Him alone), and oneness in the names and attributes of Allah.
The testimony «there is no god but Allah» is the first pillar of Islam and the essence of the call of every prophet without exception.
The opposite of tawhid is shirk: ascribing «partners» to Allah, or worshipping someone or something besides Him. It is the gravest sin in Islam.