Shaykh ul-Islām ibn Taymiyyah [may Allāh have mercy on him] said:
“Certainly, Tawheed is the quintessential element of the Qur’ān, and the essence of true faith.”
At-Tawassul wal-Waseela | Page 309 | Shaykh ul-Islām ibn Taymiyyah [may Allāh have mercy on him]
Note: Tawheed is to have belief in the Oneness and Uniqueness of Allāh, Exalted be He, with respect to His Divine Lordship and His right to be worshipped alone without ascribing any partners unto Him with respect to His Divine Names and Attributes.