I’ve heard on more than one occasion from Shaykh Ahmad and Shaykh Seraj, that Difference of opinion is a Mercy. I find this quote to be a piece of wisdom which changes views, mindsets, perception and more. Below is an unbroken chain link, leading to The Shaykh of the Sufi’s, Abu Yazid al-Bistami, in which the great saint expresses his advancement in his spiritual journey, which he attributes to the difference of opinion of the scholars.
Imam Abu l-Qasim al-Qushayri narrates from Muhammad bin al-Husayn (may God have mercy on him) from Mansur bin Abdullah from ‘Ammi al-Bastami from Abu Yazid al-Bistami:
“I have engaged in spiritual struggle and ascetic exercises for thirty years and found that there is nothing harder for me than the knowledge [of the Divine Law] and adherence to it. And were it not for the difference of opinion of religious scholars, I would have remained where I was [at the beginning], for the disagreement of scholars is a [divine] blessing, except in one issue – that is, the understanding of Gods uniqueness.” (Risala of Imam al-Qushayri).
Bayhaqi and Zarkashi also said: Qutada said: `Umar ibn `Abd al-`Aziz used to say:
“I would dislike it if the Companions of Muhammad did not differ among them, because had they not differed there would be no leeway (for us).” 
Bayhaqi also relates in al-Madkhal and Zarkashi in the Tadhkira: Al–Layth ibn Sa`d said on the authority of Yahya ibn Sa`id:
“The people of knowledge are the people of flexibility (tawsi`a). Those who give fatawa never cease to differ, and so this one permits something while that one forbids it, without one finding fault with the other when he knows of his position.” 
It is agreed upon that difference of opinion does not occur in Qat’i matters while it occurs on almost every single point in Dhanni matters.
Some examples of qatʻi matters:
The oneness of God, The finality of Prophet Hood, The prohibition of unlawful murder, The prohibition of homosexuality, The prohibition of adultery/fornication, The prohibition of drinking alcohol, The obligation of the five daily prayers, Declaring a Muslim of being guilty of Shirk without verification or even asking, ‘Please explain to me what it is you are doing or what your intentions are’.
The last Qat’i point mentioned above is a common one used, which I believe stems from the mentality of the Khawarij.
Some examples of dhanni matters:
The permissibility of taking usurious loans in non-Muslim countries, The obligation of the face veil (niqab), The permissibility of gelatin/animal rennet and other animal derivatives, What Allah means when he mentions hands, face, etc. in the Quran, Women travelling without a chaperone, The validity of wiping over cotton socks in wuḍu, The permissibility of music, The permissibility of Milad celebrations, The issue of women singing in front of non-maḥariim, Men wearing clothes below the ankles, The ruling of the beard and its length etc.
Throughout history, certain schools held and hold the position and view that the difference of opinion in Dhanni matters is nothing but a curse, which divides the community. Such groups hold those who perform external acts of worship like the ablution, prayer, dress code, etc, in a manner which differs from the way they perform these acts to be either a deviant (tafsiq), guilty of innovation (tabdi) or even blasphemy (takfir).
This manner of thought goes directly against the Sunnah of Allah and Sunnah of Muhammad (Peace be upon him) as clearly indicated in so many narrations.
The word ‘tolerance’ means ‘to bear’. It means ‘respect, acceptance and appreciation of the rich diversity of the world’s cultures, forms of expression and ways of being human’. In Arabic it is called ‘tasamuh’. There are also other words that give similar meanings, such as ‘Hilm’ (forbearance) or `afu’ (pardon, forgiveness) or ‘safh’ (overlooking, disregarding).
Tolerance is a basic principle of Islam and it is a religious moral duty. Allah says,
‘O mankind! We have created you from one man and one woman, and have made you into various nations and tribes so that you may know one another’.
From this verse it is clear that the diversity in creation is inherent and by divine design. Its purpose is to teach us to interact, to get to know one another, to learn from one another and to be a part of the global picture. The many different cultures that exist in the world today are a blessing.
The narrations on tolerance are too many to mention, we will quote but a few:
The Messenger of Allah, peace be upon him, said,
“This Qur’an has been revealed in seven different ways.”
There are so many interpretations of what seven ways actually mean. Ibn Hibban, May Allah be pleased with him states there are approximately thirty-five different opinions with regards to the understanding of the seven different ways. 
Jabir reported: The Messenger of Allah, peace be upon him, said,
“Allah has forgiven a man who was before you: he was tolerant when selling, tolerant when purchasing, and tolerant when repaying.” 
Muhammad (Peace be upon him) used to visit the sick people among the Jews as well as the Muslims and when on one occasion the funeral procession of a Jew passed before him, he stood up as a sign of respect for the deceased. ‘Why did you stand up for a Jewish funeral’, they asked. The Prophet replied,
‘Is it not a human soul’?
We have four accepted schools of thought/madhahib, namely, the school of Imam Malik, Abu Hanifa, al-Shafi’i and Imam Ahmad bin Hanbal. Within each of the mentioned schools of thought, we have scholars who differed with the founder of the school. If Imam Shafi’i said something then we would have to say that ‘Imam Shafi’i said so and so’. If scholars within the school said something then we would have to say for example, ‘Imam Ghazali or ibn Hajar or Imam Nawawi of the Shafi’iya said so and so’.
If one scholar per school differed with the founder of the school, we would have not four, but a minimum of eight different opinions on certain dhanni acts.
I will list only one difference between The Shafi’iya and Abu Hanifa.
The greeting of As-salamu ‘alaykum at the end of the prayer
According to Imam Shafi’i, greeting to the right or the first greeting is an integral/rukun of the prayer while greeting to the left or second greeting is a sunnah or more specifically, it is called a hay-ah by the Shafi’iya. According to the Shafi’iya, if one omits a hay-ay then the prayer is still deemed as valid.
The Shafi’iya use as evidence for this ruling the hadith in which the Prophet (Peace be upon him) said,
‘The tahrim of the salat is the takbir, and its tahlil is the taslim’
Meaning after the takbir, it is now made haram (tahrim) to talk and do things that are extraneous to the salat since it has commenced, and after the taslim it is now made halal (tahlil) as the prayer has finished, so the tahrim and tahlil are used to refer to its beginning and end. 
According to Abu Hanifa, greeting to the right and left is considered an obligation/wajib and not an intergral/rukun. Abu Hanifa relies on the hadith of ‘Abdullah ibn Mas’ud (may Allah be pleased with him):
Alqamah said that Abdullah ibn Mas’ud caught hold of his hand saying that the Apostle of Allah (Peace be upon him) caught hold of his (Ibn Mas’ud’s) hand and taught him the tashahhud during prayer.
He then narrated the (well known) tradition (of tashahhud) in which the Apostle of Allah (Peace be upon him) said,
‘When you say this or finish this (meaning the tashahhud), then you have completed your prayer. If you want to stand up, then stand, and if you want to remain sitting, then remain sitting. 
Why do the Imams differ on this point?
For an act to be categorized as an Integral/Rukun by the Hanafiya, the minimum is that the hadith must be on a level called Mash-hur. 
The Hadith used by the Sahfi’iya is according to the Hanafiya, on a level less authentic called khabar wahid . The Hanafiya do not declare an act to be an integral based on an Ahad or khabar wahid narrations. The Hadith used by the Shafi’iya according to Imam Farghani al–Marghinani of the Hanafiya, in his worked entitled al-Hidaya, is a khabar wahid. 
This is then an example of how and why the schools of thought differ. I pray that this example sheds some light into the realm of difference of opinion of the scholars. It is my view that because such difference of opinion, that if one were to see the next person doing something different to what he is familiar with, then by default, the difference is correct and bad thought of him being wrong is in itself wrong.
And Allah knows best.
1) Habib Husayn ibn Muhammad al-Hibshi, the son of Sayyid Muhammad bin Husayn al-Hibishi. Habib Husayn ibn Muhammad al-Hibshi was the contemporary of the founder of the Zawiya, Shaykh Muhammad Salih Hendricks.
2) Al-Jarrahi cited it in Kashf al-khafa 1:64 #153.
3) Ibn Taymiyya, Mukhtasar al-fatawa al-misriyya (Cairo, 1980) p. 35, 54.
4) Sahih al-Bukhari, hadith 4992.
5) Fathu l-Bari, ibn Hjar Asqalani.
6) Sunan At-Tirmidhi, Book of Business, Number 1320.
7) Fiqh al-Sunnah, Sayyid Saqib.
8) Sunan Abu Daud, volume 1, page 403, chapter of Tashahud, tradition # 957, translated by Maulana Waheed uz-Zaman, published by No’mani book store, Lahore.
9) See https://islamictext.files.wordpress.com/2012/05/does-touching-the-opposite-sex-break-ones-wudhu1.pdf for more details on Mash-hur and Ahad narrations.
10) AL-Hidaya, Book of Prayer, Imam al-Marghinani.