Answer: Not necessarily. For example:
1. Quran is Nur – sura Maida (5):15 - قَدْ جاءَكُمْ مِنَ اللَّهِ نُورٌ وَ كِتابٌ مُبِين - Certainly there has come to you a light from Allah, and a manifest Book
2. Allah is the Nur and Light - sura nur (24):35 - اللَّهُ نُورُ السَّماواتِ وَ الْأَرْض - Allah is the nur and light of the heavens and the earth, meaning He is the Guide
3. Faith and Iman is light and Nur – sura baqara (2): 257 – اللَّهُ وَلِيُّ الَّذِينَ آمَنُوا يُخْرِجُهُمْ مِنَ الظُّلُماتِ إِلَى النُّور - Allah is the Guardian of the faithful: He brings them out of darkness into light
4. Divine Guidance is Nur – sura An’aam(6):122 - أَ وَ مَن كاَنَ مَيْتًا فَأَحْيَيْنَاهُ وَ جَعَلْنَا لَهُ نُورًا يَمْشىِ بِهِ فىِ النَّاس - Is he who was lifeless, then We gave him life and provided him with a light by which he walks among the people
5. The holy prophet is Nur – sura Ahzab(33):46 - وَ دَاعِيًا إِلىَ اللَّهِ بِإِذْنِهِ وَ سِرَاجًا مُّنِيرًا - and as a summoner to Allah by His permission, and as a radiant lamp
6. The Imams are the Nur – as we read in Zyarat Jamea Kabira - خَلَقَكُم اللَّهُ اَنوَارَا فَجَعَلَكُم بِعَرشِهِ مُحدِقِين - Allah created You (plural) as light, and made you circle His throne
7. Knowledge is Nur – hadith - اَلعِلمُ نُورٌ يَقذِفُهُ اللَّهُ فِى قَلبِ مَن يَشاءُ – knowledge is light, that Allah casts into the heart of the one He wants to.
Examples 1-5 are from the qur’an and they are easily understandable, and have no direct relationship with Imam Ali (alayhis salam). When Allah is nur, that means here it is Allah who is nur not Ali.
Having said all that, let us consider the light/nur that we can feel around us: if it weren’t for light, we would not see anything! If it weren’t for (sun)light we wouldn’t be able to survive and grow…
When we, therefore, apply this word ‘nur’ to Allah, we see nur to be the most perfect example of His Being, ie if it weren’t for Him, nothing would exist, it is He who exists and gives existence to everything else! If it weren’t for Him, we wouldn’t be able to survive, grow and be guided to Perfection.
In conclusion, we can apply this nur of Allah to the Ma’sumeen (alahimus salam) and especially Amirul Mumineen and say: he is the keeper of the religion of Allah after the holy prophet, he is guided and he guides others…
So the word nur with its apparent meaning from the Quran can be applied to many things, but its highest meaning is that of the light of Allah, and the Ma’sumeen (alayhimus salam) are the exemplars (the role models and those who carry out Allah’s commands to the most perfect level) of that light. So the inner meanings of the Qur’an point to Imam Ali (alayhis salam) as the nur from this angle.
2. So when the sheikhs are saying that all the verses of the Qur’an in mentioning nur, mean Ali (alayhis salam) it is with the inner meaning of the verse, and there is nothing wrong with that as his nur is the nur obtained from Allah.
3. As mentioned above, the Qur’an has also been called nur, i.e. the one that guides from darkness to light. This guidance would start with the commencement of the prophethood (at 40yrs age). Before that time, Muhammad (allahumma salli ala muhammadin wa ali Muhammad) was not commanded to guide the people to islam. If Imam Ali (alayhis salam) has to be nur, then with the meanings suggested above he would be a guide of mankind after the prophet’s demise.
With regards to the stamp of prophethood on the prophet’s back, there is difference of opinion as to how it was and when it appeared. Secondly, it was not the whole quran, but rather a part of a verse. Allama Majlisi in Bihar ul Anwar (vol 16, pg 177) mentions that the words ‘There is no god but Allah, the One, who has no partner, face any direction you want, you are the helped one’ were stamped on his back.
This stamp was more of a way of confirming his prophethood, and not with the meaning that it is nur that the Quran mentions.
I would suggest reading tafsir of the verses containing the word nur to get better understanding of the subject, inshaAllah