Answered by Mufti Abdullah al-Ma’mun
Question:
Assalamu Alaikum Mufti Saheb. If someone from the followers of any of the four Imams says that to follow Imam Abu Hanifa is wrong because he has misguided the Muslims then can he still remain in Iman or not?
Answer:
Imam Abu Hanifa 
is considered to be a tabi’ee (someone who met a companion of Allah’s messenger 
or merely saw one while in the state of Iman), and this has been confirmed by many biographers and historians. This is a unique position held by him, since the same cannot be said regarding the other great Imams, Shaf’ee, Malik and Ahmad ibn Hanbal . His depth of knowledge in Qur’an and Hadith, his trustworthiness, his honesty, piety and his fear of Allah were testified to by thousands of Imams and scholars of the ummah.
Hence, to make these kinds of allegations and pass such comments regarding such a great individual is indeed frowned upon and must be refrained from. Such people do nothing besides expressing their own lack of knowledge and are advised to seek repentance lest they should become amongst those people regarding whom Allah’s messenger said in a lengthy hadith: “Amongst the signs of the hour are [. . .] when the latter part of the ummah will speak ill of the former part of the ummah”. May Allah Ta’aala protect us aameen.
(Baihaqi Shu’ab al-Iman vol:1 pg:307)
However, despite the seriousness of this statement, a person will not become a disbeliever by saying it, as he is not openly opposing any of the primary branches of Islam. Making takfeer (declaring a disbeliever or apostate) of a Muslim is a very delicate issue and the fuqaha (Jurists) have stressed that it must be avoided at all costs.
(See al-Durr al-Mukhtar pg:345 and Shaami Bab al-Murtad vol:16 pg:256 and Jawaahir al-Fiqh vol:1 pg:36)
Only Allah Knows Best
Written by Mufti Abdullah al-Ma’mun
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham.