Shaykh ul-Islām ibn Taymiyyah [may Allāh have mercy on him] said:
“The essence of the religion is to encourage what is good, and forbid what is evil. The pinnacle of goodness 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 and the pinnacle of evil is associating partners unto Allāh.”
Majmū’ al-Fatāwa, 27/442 | Shaykh ul-Islām ibn Taymiyyah [may Allāh have mercy on him]