In Islamic history, the door of ijtihad (independent reasoning about religious matters at an individual level) has always been open.
Reasoning and seeking spiritual knowledge is applauded in Islamic belief in general, and to open the door of ijtihad there are certain laws to follow.
There are levels of Islamic religious knowledge that starts off with alem (seeker of Islamic knowledge), then faqih (excellence in Islamic knowledge) then imam or a leader that owns a specific school of thought.
Imam is the highest rank because being the author of a school of thought requires a lot of credentials and studies, in order to be able to define and implement the divine laws in different situations.
Throughout the history of Islamic civilisation, four imams and their schools of thoughts were the most dominant when it comes to Islamic legislation (sharia and fiqh): Imam Abu Hanifa Al-Noman (699-767 AD), Imam Malek Ibn Anas (715-796 AD), Imam Mohamed Ben Idris El-Shafai (766-820 AD), Imam Ahmed Ibn Hanbal (699-767 AD).
They were all people of immense religious knowledge and though they all agreed on the main and core Islamic beliefs, they disagreed on some Islamic legislations and there is a popular saying: ikhtelafhom rahma (“their differences are a blessing”) because one can pick and choose from the school of thought that suits one best.
We start with the first Imam, Abu Hanifa Al-Noman.
Born in 699 AD in Kufa, Iraq, to a silk merchant, Selman Faiad states in his book The Four Imams, "when Abu Hanifa's father met Imam Ali Ibn Abi Taleb, he offered Imam Ali some candy, which was a delicacy, in celebration of the Persian festival of Al-Nairouz. Then Imam Ali put his hands on his head and prayed for prosperity for him and all his descendants.”
As a child, Abu Hanifa memorised the holy Quran. He grew up in a very cosmopolitan town where different ethnicities, including Arabs and Persians, lived in peace, where Greek philosophy met with the wisdom of Persia.
There was endless cultural dialogue between different sects of Christianity and the different Islamic sects -- Shiaa, Sunna, Khawarij among others, and a lot of political variations as well.
According to the book, Abu Hanifa's input is some 83,000 rules from sharia, out of which 38,000 are to do with the essence of worship, and 45,000 about the logistics.
A couple of his most famous quotes:
قال أبو حنيفة لأبي يوسف : ( إياك أن تكلم العامة في أصول الدين من الكلام ، فإنهم قوم يقلدونك فيشتغلون بذلك ) [ مناقب أبي حنيفة للمكي 373]
"Never discuss with the commoners [those who do not have sufficient religious knowledge] the essence of religion, for they can imitate you and proclaim themselves imams."
"God never forced any of his creations to believe or disbelieve in him. He created them as individuals, and faith or disbelief is a human act. Moreover, God knows who believes in him and who does not. He loves those who believe in him regardless of their past disbelief."
In his lifetime the imam survived two grave atrocities, the first during the Ummayed period because he sided with the revolution of Imam Zeid Ibn Ali and refused to work under the auspices of the wali of Kufa, Yazid Ibn Omar Ibn Habira.
So he was beaten, locked up, but then managed to flee to Mecca in 747 AD until the Islamic caliphate fell into the hands of the Abbasids; then he returned to Kufa during the reign of Abbasid caliph Abi Gaafar Al-Mansour.
The second ordeal he faced was during the reign of the Abbasid caliph, where he sided with revolution of Imam Mohamed Nafs Al-Zakia, and would voice his objections to some of Al-Mansour’s intentions, and even refused to be the grand judge during his reign.
Al-Mansour detained him until he died in Baghdad in 767 AD and he was buried in Baghdad, where a mosque was built in his honour later.