In Surat Al Ma'ida (verse no. 3) Allah (SWT) says "..This day I have perfected/completed for you your religion, and completed my favour upon you and accepted Islam as your religion.."
So what can we take from this verse?
1- Allah (SWT) states very clearly that the religion/way of life (Islam) has been completed/perfected at the time of it's revelation. This verse was revealed to Prophet Muhammad ﷺ during his delivery of the Farewell sermon in the year 631 CE.
What this means is that we should not accept any innovation (Bid'ah) in Islam, it is complete and no addition or subtraction is necessary or acceptable
The Prophet ﷺ said:
"…and the best speech is the Speech of Allah, and the best guidance is the guidance of Muhammad ﷺ, and the worst of all affairs are the newly invented matters (in the religion)”
“He who innovates something that is not in agreement with our matter (Islam), will have it rejected.” (Al-Bukhari and Muslim)
"..Beware of newly invented matters, for every invented matter is an innovation and every innovation is a going astray, and every going astray is in Hell-fire.”
(Abu Dawud & Al-Tirmidhi)
2-Allah (SWT) says He has completed His Favour upon us and His favour is Islam i.e. the Quran and the Sunnah of Prophet Muhammad ﷺ. Allah (SWT) has indeed given us a great favour by giving us Islam and a way/path to attaining His mercy on the day of Judgement.
Allah (SWT) has sent mankind many prophets & messenger to guide and warn us, He has given us the tools needed to defend ourselves from the accursed shaytan and to build a hereafter in Jennah. Indeed Islam is the greatest favour and blessing for mankind.
3-Finally Allah (SWT) says He has chosen & accepted Islam as out religion/way of life. This is an amazing statement. If we understand that Jannah is not something simple or cheap and Allah deserves so much more that we can give. Yet Allah has made Islam easy/achievable and He accepts this religion from us and in return has given us everything and this too is an amazing favour.
This once verse tells us so much and shows us that Allah is truly compassionate and merciful.
Narrated by Omar bin Al-Khattab (raa): " Once a Jew said to me, "O the chief of believers! There is a verse in your Holy Book Which is read by all of you (Muslims), and had it been revealed to us, we would have taken that day (on which it was revealed as a day of celebration." 'Omar bin Al-Khattab asked, "Which is that verse?" The Jew replied, "This day I have perfected your religion For you, completed My favor upon you, And have chosen for you Islam as your religion." (5:3) 'Omar replied,"No doubt, we know when and where this verse was revealed to the Prophet. It was Friday and the Prophet was standing at 'Arafat (i.e. the Day of Hajj)"
As Muslims this verse is a blessing and for this we must be grateful servant inShaa Allah.