Make God your Sole Concern
Concentrating On Calling People To Tawheed (Singling Out Allaah For Worship)
Verily the greatest and the most important issue that one should be concerned with is the issue of 'aqeedah; the issue of tawheed and it's opposite.
Tawheed is the reason that Allaah, the Most High, sent the Messengers, revealed the Books, and created mankind and jinn. The other regulations in Islaam are all secondary to tawheed.
Allaah says in the Qur'aan,
وَمَا خَلَقۡتُ ٱلۡجِنَّ وَٱلۡإِنسَ إِلَّا لِيَعۡبُدُونِ
I have not created jinn and mankind except to worship me.
(Soorah adh-Dhaariyaat (51):56)
[Book: Words Of Advice Regarding Da'wah, p. 31 | By Shaykh 'Abdul 'Azeez ibn 'Abdullaah ibn Baaz | Compiled by Ziyaad ibn Muhammad as-Sa'doon | Translated by Bint Feroz Deen & Bint 'Abd al-Ghafoor | Published by Al-Hidaayah Publishing and Distribution]