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
The da’wah of all the prophets was first and foremost to tawheed, and nothing else–not social injustice, not economic superiority, but tawheed.
Shaykh Yasir Qadhi says, the BEST way to increase your eman is to increase your knowledge of tawheed–Allah’s names and attributes, Heaven and Hell, and so on.