The highest and best obligation decreed by Allah is having the correct belief in God and His Messenger. The Prophet, salla-l-lahu ^alayhi wa sallam, said:

أفضل الأعمال إيمان بالله ورسوله” رواه البخاري”

This means: “The best of deeds is to believe in Allah and His Messenger,” narrated by al-Bukhariyy. 

Having the correct Islamic 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,

﴿مثل الذين كفروا بربهم أعمالهم كرماد اشتدت به الريح في يوم عاصف﴾ سورة إبراهيم

This verse means: [The example of those who disbelieve in their Lord (Allah) is that their deeds are like ashes, which the wind blows forcefully on a stormy day].