The highest and best obligation as decreed by Allah is: The Belief in Allah and his messenger. The prophet, salla-l-lahu ^alayhi wa sallam, said:
أفضل الأعمال إيمان بالله و رسوله
This means: The best of deeds is to believe correctly in Allah and his messenger (narrated by al-Bukhariyy). Having the correct Belief is a condition for the acceptability of the good deeds. Hence, the one who does not believe in Allah and his messenger will never have any reward in the Hereafter. Allah said in Surat Ibrahim verse 18 which means: The deeds of those who blasphemed is like dust which was scattered by wind in a stormy day .
Be advised that the knowledge of Tawhid has a privileged merit over other types of knowledge. This is because it relates to the most honored information. The high honor of the knowledge is due to the high status of the things one knows about. Since the knowledge of Tawhid conveys the knowledge about Allah, clears Allah of what is not befitting to Him and clears the prophets of what is not befitting to them, it is better than the knowledge of the rulings. Allah said in Surat Muhammad verse 19
{ فاعلم أنه لا إله إلا الله و استغفر لذنبك }
which means know (be steadfast) O’ Muhammad that no one is God except Allah and ask Allah for forgiveness.
Imam Abu Hanifah said in his book <Al-Fiqh al-’Absat>: “Know that the knowledge of the essentials of the Religion is better than the knowledge of rulings.”