According to the consensus of the majority of commentators, the Sahabah and the Tabi'in, it means the Holy Qur'an which was sent for the universal community of inviters to faith including the Jews and Christians through the (midmost) medium of the Holy Prophet (SM). Therefore, the verse comes to mean 'unless you act in accordance with the injunctions brought by the Torah, the Injil and the Qur'an correctly, fully and totally no lineal or intellectual excellence of yours shall be valid or acceptable with Allah.'
At this point one may wonder why, in this verse, rather than mentioning Qur'an by its brief name, as was done with the Torah and the Injil, a long sentence : وَمَا أُنزِلَ إِلَيْكُم مِّن رَّبِّكُمْ (and what has been revealed to you from your Lord) has been used. What is the wisdom behind it? It is possible that it may be carrying a hint to what was said by the Holy Prophet (SM) in which he disclosed that the way he was given the Qur'an which is a treasure of knowledge and wisdom, also given to him were other sources of knowledge and insight, which could be called, in a certain aspect, the explication of the Holy Qur'an as well. The words of the Hadith are:
Remember! I have been given the Qur'an and, with it, (many fields of knowledge) like it lest, in times to come, some sated man reclining on his couch were to say: For you this Qar'an is sufficient what you find Halal in it, take that as Halal; and what you find Haram in it, take that as Haram although, what the Messenger of Allah (SM) has declared to be Haram is like what Allah has declared to be Haram. (AbL DavrLd, Ibn Majah, Darimi & others).
Three Kinds of Ahkam
The Qur'an itself is a testimony of this subject. It says: وَمَا يَنطِقُ عَنِ الْهَوَىٰ إِنْ هُوَ إِلَّا وَحْيٌ يُوحَىٰ which means that the Holy Prophet (SM) does not say anything on his own whatever he says is all revealed to him from Almighty Allah; and under conditions when he says something based on his Ijtihad (fully deliberated judgement) and Qiyas (analogical deduction) following which he receives no guidance contrary to it then, consequently, that Qiyas and Ijtihad too hold the same legal force as that of an injunction revealed through Wahy.
In a nutshell, we can say that, out of the injunctions (Ahham) given to the Ummah by the Holy Prophet (SM), first come those which have been mentioned clearly in the Holy Qur'an. Then, there are those which have not been mentioned in the Qur'an clearly, instead of which, they were revealed to the Holy Prophet (SM) through a separate Wahy (Wahy Ghayr Matlu Revelation not recited). Thirdly, the command which he gave as based on his Ijtihad and Qiyas following which no command was revealed by Almighty Allah then, that too takes the legal force of a revealed injunction. It is obligatory to follow all these three kinds of injunctions. They are included within the purview of : وَمَا أُنزِلَ إِلَيْكُم مِّن رَّبِّكُمْ (What has been revealed to you from your Lord).
It is also possible that the use of the longer sentence: وَمَا أُنزِلَ إِلَيْكُم مِّن رَّبِّكُمْ (and what has been revealed to you from your Lord) in place of the likely brief name of Qur'an in this verse may have been preferred so as to indicate that the following of all injunctions given in the Qur'an given by the Holy Prophet (SM) equally mandatory.
Another point worth noting in this verse is that the Jews and christians have been asked to follow the injunctions of all three Books, that is, the Torah, the Injil and the Qur'an although, some of them abrogate others. The Injil abrogates some injunctions of the Torah and the Qur'an abrogates many injunctions of the Torah and the Injil. How then, could the sum of three be followed in action?
The answer is clear. Since every incoming Book changed some injunctions of the outgoing Book, it becomes obvious that following the replaced modality, in itself, amounts to acting in accordance with both Books. Following and acting in accordance with abrogated injunctions is against the requirement of both Books.
Allah consoles His Prophet
To console the Holy Prophet (SM) it was said at the end that there will be many among the people of the Book who would fail to benefit by the Divine magnanimity shown to them. They may even react adversely by increasing the tempo of their disbelief and hostility. Let this be no cause of grief to him, nor should he have any sympathetic concern for them.
Salvation promised for Four Communities
having faith and good deeds
In the second verse (69), Allah Almighty addresses four traditional religious communities, pursuades them to have faith and act righteously whereupon He promises salvation for them. The first of those are: الَّذِينَ آمَنُوا'(those who believe) that is, the Muslims. Secondly : الَّذِينَ هَادُوا (and those who are Jews); thirdly : صَّابِئُونَ (the Sabians); and fourthly : لنَّصَارَىٰ is (the Christians). Three of these Communities Muslims, Jews and Christians are well-known and found in most parts of the world. Any community having the name Sabian does not exist today. Therefore, scholarly opinion varies as to their exact identity. Quoting Qatadah, the Tafsir authority, Ibn Kathir has reported that Sabians were people who worshipped angels, offered prayers contra-oriented to the Qiblah and recited the Scripture, Zabur (revealed to Sayyidna Dawud) عليه السلام.
The context of the Qur'an seems to support it as the four Scriptures mentioned in the Qur'an are Torah, Zabur, Injil and Qur'an. Thus, named here are the believers in these four Books.
Another verse on the same subject, almost in the same words, has appeared in the seventh Section of Surah al-Baqarah:
إِنَّ الَّذِينَ آمَنُوا وَالَّذِينَ هَادُوا وَالصَّابِئُونَ وَالنَّصَارَىٰ مَنْ آمَنَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ وَعَمِلَ صَالِحًا فَلَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ
Surely, those who believed, and those who are Jews, and Christians,and the Sabians whoever believes in Allah and in the Last Day, and does good deeds they have their reward with their Lord. And there is no fear for them nor shall they grieve. (2: 62)
Other than the place-oriented transposition of some words, there is no difference between them.
With Allah, Distinction Depends on Righteous Deeds
The gist of both these verses is that distinctions of lineage, homeland, and nationality do not matter in the sight of Allah. Anyone who takes to total obedience, faith and good deeds as a way of life no matter what he has been before shall find himself acceptable with Allah and his devotion will bring the best of appreciation from his Creator. And it is obvious that total obedience after the revelation of the Qur'an depends on being a Muslim because, there are instructions to this effect in the past Scriptures of the Torah and the Injil as well, while the Qur'an itself was revealed for this particular purpose. Therefore, after the revelation of the Qur'an and the appearance of the Last among Prophets (SM) it cannot be correct without believing in the Qur'an and having faith in the Holy Prophet (SM) to follow either the Torah or the Injil or the Zabir. Thus, these verses would mean that whoever from among these communities becomes Muslim shall become deserving of salvation and reward in the Akhirah (H-ereafter). Provided right here is the answer to the doubt as to what would happen to all their dark doings of sin and disobedience and disbelief and anti-Islam and anti-Muslim mechanizations once they have become Muslims? The disarming answer is: All past sins and shortcomings will stand forgiven and in the life-to-come,they shall have no fear or grief.