So verily the knowledge of tawheed [the oneness of Allaah] is the noblest of sciences, and the greatest in rank, and most obligatory to be studied because it is the knowledge of Allaah the Exalted, and His Names, and His Attributes, and His Rights upon His slaves.
Shaykh Muhammad Saalih Ibnul ‘Uthaymeen
So this [tawheed] is the most fundamental principle, and the greatest understanding. It is the first thing that the writers should write about, and it is what the callers to guidance, and what the supporters of the truth should focus on. It is the most important science that they should grab onto with their molar teeth, and it should be spread amongst all societies, until they know its reality, and until they distance themselves from whatever opposes it.
Shaykh ‘Abdul ‘Azeez Ibn ‘Abdullaah Ibn Baaz