Commentary
In verse 2, it was said: فَاعْبُدِ اللَّـهَ مُخْلِصًا لَّهُ الدِّينَ أَلَا لِلَّـهِ الدِّينُ الْخَالِصُ - (so worship Allah in submission to Him exclusively. Remember, Allah alone deserves the exclusive submission.). The word: دِین (deen) at this place means worship, submission, devotion or obedience which is inclusive of adhering to all religious injunctions. In the first statement earlier to this, the address was to the Holy Prophet ﷺ where he was commanded to make the worship of and obedience to Allah sincere and exclusive for Him, so as there remains no trace of shirk or of any hypocrisy or false pretension. The second statement is to emphasize the first statement in that sincerity in worship is the exclusive right of Allah and no one else, other than Him deserves it.
It has been reported from Sayyidna Abu Hurairah ؓ that someone submitted before the Holy Prophet ﷺ ، 'Ya rasulallah, there are occasions when I give in charity or do a favor to somebody, but my intention gets mixed up - I do look forward to seek the pleasure of Allah thereby, but I also hope that people will praise me for it.' The noble prophet of Allah ﷺ said, "By the One in whose hands lies the life of Muhammad, Allah Ta’ ala does not like anything in which someone or something else has been associated as a partner or sharer with Him. Then he recited the verse quoted here as the proof: أَلَا لِلَّـهِ الدِّينُ الْخَالِصُ (Remember, Allah alone deserves the exclusive submission.) ". (Qurtubi)
Acceptance of deeds with Allah depends on unalloyed measure of sincerity
Several verses of the Qur'an bear out that the deeds are not measured with Allah by numbers, instead, they are measured by weight, for example: وَنَضَعُ الْمَوَازِينَ الْقِسْطَ لِيَوْمِ الْقِيَامَةِ (and We shall place scales to do justice on the Day of Judgment - Al-Anbiya', 21:47). This verse read in juxtaposition with the verses cited above tell us that the value and weight of deeds is judged by the amount of sincerity with which they are performed, and it goes without saying that perfect sincerity cannot be achieved without perfect faith, because perfect sincerity means that one should not take someone as having intrinsic power to benefit or harm someone other than Allah, nor take someone other than Allah as disposer of one's acts, nor allow the thought of someone other than Allah enter into any act of worship or obedience of his volition. As for non-voluntary scruples, Allah Ta’ ala forgives them.
The noble Sahabah are the front line of Muslims. Their practice of prayers and good deeds may not appear to be very prominent numerically. But, despite all this, the reason why the single insignificant most of their deeds was superior to the highest of the high deeds of the rest of the Ummah was no other but their very perfection of 'iman (faith) and perfection of ikhlas (sincerity).