وَإِذَا قِيلَ لَهُمُ ٱتَّبِعُواْ مَآ أَنزَلَ ٱللَّهُ قَالُواْ بَلۡ نَتَّبِعُ مَآ أَلۡفَيۡنَا عَلَيۡهِ ءَابَآءَنَآ‌ۗ أَوَلَوۡ كَانَ ءَابَآؤُهُمۡ لَا يَعۡقِلُونَ شَيۡـًٔ۬ا وَلَا يَهۡتَدُونَ O (سورة البقرة:۱۷۰)

AND WHEN IT IS SAID TO THEM: "FOLLOW WHAT ALLAH HAS SENT DOWN," THEY SAY: "INSTEAD, WE WOULD FOLLOW WHAT WE FOUND OUR FATHERS ON."IS IT SO - EVEN THOUGH THEIR FATHERS USED TO UNDERSTAND NOTHING, NOR HAD THEY BEEN ON THE RIGHT PATH? (SURAH AL-BAQARAH:170)

This verse not only censures the blind following of forefathers, it also sets out a rule and its attendant conditions, for Taqleed (following) which has been hinted at by the words لَا يَعۡقِلُونَ (even though their forefathers used to understand nothing) and لَا يَهۡتَدُونَ (nor had they been on the right path). This tells us that following these forefathers was forbidden because they neither had reason nor guidance. 'Guidance' or the right path signifies injunctions revealed by Allah Ta'alah, openly and clearly, while 'aql' or reason stands for imperative guidance deduced through Ijtihad from the recognised sources of Islamic Shariah. So the reason it is not permitted to follow these ancestors is that they do not have a revealed set of laws from Allah, nor do they have the ability to deduce injunctions from the Word of Allah. This leads to the deduction that if there is such an Alim (scholar) who has  good knowledge of Quran and Sunnah, and in the absence of a clear and direct instruction of the two sacred sources has the ability to derive rulings through Ijtihad then it is permissible to follow such a Mujtahid Alim. The purpose is not to follow commands of the Alim himself, the idea is to follow the commands of Allah Ta'alah, but if a person does not have complete knowledge of Quran and Sunnah, and does not have the ability to derive laws from Quran and Sunnah, then what valid option has he got left but to follow an authentic Alim who does have such knowledge and ability.

In the interpretation of this verse Imam Al-Qurtubi RE has said that the prohibition of following forefathers mentioned in this verse applies to following them in false beliefs and deeds. Following of correct beliefs and good deeds is not included in the meaning of this verse as it has been very clearly said in Surah Yusuf in the words of Hazrat Yusuf AS:

إِنِّى تَرَكۡتُ مِلَّةَ قَوۡمٍ۬ لَّا يُؤۡمِنُونَ بِٱللَّهِ وَهُم بِٱلۡأَخِرَةِ هُمۡ كَـٰفِرُونَ O وَٱتَّبَعۡتُ مِلَّةَ ءَابَآءِىٓ إِبۡرَٲهِيمَ وَإِسۡحَـٰقَ وَيَعۡقُوبَ‌ۚ

"I have disassociated myself from the community of people who do not believe in Allah and who deny the Hereafter and I have chosen to follow the community of my fathers, Ibrahim and Ishaq and Yaqub AS. (Surah Yusuf:37-38)

Adapted from Ma'ariful Quran, by Mufti Muhammad Shafi RE