Tafseer of the verse “Thus We have made you a just (and the best) nation”
Question

Allah says: “Thus We have made you a just (and the best) nation”. What is meant by ummatan wasataa (translated here as “a just (and the best) nation”? What is meant by bearing witness over mankind?.
Praise be to Allah.Allah says (interpretation of the meaning): 
“Thus We have made you [true Muslims — real believers of Islamic Monotheism, true followers of Prophet Muhammad and his Sunnah (legal ways)], a just (and the best) nation, that you be witnesses over mankindand the Messenger (Muhammad) be a witness over you”  [al-Baqarah 2:143]
There are saheeh ahaadeeth from the Prophet (peace and blessings of Allah be upon him) which explain this verse and state that what is meant by ummatan wasata is just and best, and what is meant by bearing witness over mankind is bearing witness against the nations on the Day of Resurrection, that their Messengers conveyed the Message to them. The words of the mufassireen do not refer to anything other than that. 
Al-Bukhari (4487) narrated that Abu Sa’eed al-Khudri (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Nooh (peace be upon him) will be called on the Day of Resurrection and it will be said to him: ‘Did you convey (the message)?’ He will say: ‘Yes.’ Then his people will be called and it will be said to them: ‘Did he convey (the message) to you?’ They will say: ‘No warner came to us and no one came to us.’ It will be said to Nooh: ‘Who will bear witness for you?’ He will say: ‘Muhammad and his ummah.’” He said: “That is the words of Allah, ‘Thus We have made you a just (and the best) nation.’” Ahmad (10891) added: He said: “And they will be called and they will bear witness that he conveyed (the message), then I will bear witness for you.” 
Imam Ahmad (1164) and Ibn Maajah (4284) narrated that Abu Sa’eed al-Khudri (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “A Prophet will come on the Day of Resurrection accompanied by one man, and a Prophet will come accompanied by two men, or more than that. Then his people will be called and it will be said to them: ‘Did this one convey the message to you?’ and they will say, ‘No.’ It will be said to him: ‘Did you convey the message to your people?’ and he will say: ‘Yes.’ It will be said to him: ‘Who will bear witness for you?’ He will say: ‘Muhammad and his ummah.’ So Muhammad and his ummah will be called, and it will be said to them: ‘Did this one convey the message to his people?’ They will say: ‘Yes.’ It will be said: ‘How did you know that?’ They will say: ‘Our Prophet came to us and told us that the Messengers had conveyed the message.’ That is the words of Allah, ‘Thus We have made you a just (and the best) nation.’ He said: Just, so that you will be witnesses over mankind and the Messenger will be a witness over you.”
Classed as saheeh by al-Albani in al-Silsilah al-Saheehah, 2448. 
Ibn Jareer al-Tabari said in his commentary on this verse: 
What this means is that just as We have made you a just nation, bearing witness for My Prophets and Messengers against their nations, that they conveyed the message to them that I commanded them to convey, so too My Messenger Muhammad (peace and blessings of Allah be upon him) will be a witness for you, that you believed in him and in what he brought to you from Me. End quote. 
Jaami’ al-Bayaan, 2/8 
Ibn Katheer said in his commentary on this verse: 
Wasata here refers to the best and most excellent, as it was said: Quraysh awsat al-‘arab nasaban wa daaran, i.e., Quraysh are the best of the Arabs in lineage and abode, and the Messenger of Allah (peace and blessings of Allah be upon him) was wasatan fi qawmihi, i.e., the best of his people and noblest in lineage. And al-salaat al-wusta means the best prayer, namely ‘Asr prayer, as is narrated in the books of Saheeh and elsewhere. 
“that you be witnesses over mankind” means: so that you will be witnesses over the nations on the Day of Resurrection, because they will all acknowledge your virtue. End quote. 
Tafseer Ibn Katheer, 1/181 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 
What we learn from this verse is the superiority of this ummah over other nations, because Allah says wasatan. 
And we learn that this ummah is just, because Allah says “that you be witnesses over mankind”, and the witness is the one whose word is accepted. 
And we learn that this ummah will bear witness over the nations on the Day of Resurrection, because Allah says “that you be witnesses over mankind.” Testimony may be given in this world and in the Hereafter. When the people are gathered and the Messengers are asked: “Did you convey the message?” they will say, “Yes.” Then the nations will be asked: “Was the message conveyed to you?” and they will say, “No bringer of glad tidings or warner came to us; no one came to us.” Then it will be said to the Messenger, “Who will bear witness for you?” He will say: “Muhammad and his ummah.” So they will be asked to bear witness on the Day of Resurrection and they will bear witness, so they will be witnesses over mankind. 
If someone were to say: How can they bear witness when they did not see? 
We say: But they heard it from one whose words are more authentic than what one sees with one’s own eyes, i.e., the Prophet (peace and blessings of Allah be upon him). End quote. 
Tafseer Soorat al-Baqarah, 2/115, 116 
Al-Baghawi narrated in his Tafseer (1/122) that al-Kalbi said: Wasatan means the followers of the wasat religion, one that represents a middle path between exaggeration and negligence, because both are blameworthy in the matter of religion.  
Shaykh al-Sa’di said in his Tafseer (p. 66): 
i.e., just and best. Allah has made this ummah just in all matters of religion, and in the middle with regard to the Prophets, between those who exaggerated about them like the Christians, and those who shunned them like the Jews, because they believed in all of them in an appropriate manner. 
And they are in the middle with regard to divine laws; they did not go to extremes like the Jews or neglect them like the Christians. 
And with regard to purity and food, unlike the Jews whose prayers are not valid except in their own synagogues and who cannot be cleansed of impurity by water, so good things were forbidden to them as a punishment; and unlike the Christians who do not regard anything as impure or forbidden, rather they permitted everything that exists. Rather the Muslims’ concept of purity and purification is the best and most perfect. Allah permitted to them (the Muslims) all good things with regard to food, drink, clothing and marriage and He forbade to them all that is impure and evil. Hence this ummah has the most complete religion, the noblest attitude and the best of deeds. Allah has bestowed upon them knowledge, forbearance, justice and kindness  such as He did not bestow upon any other nation. Hence they are ummatan wasata, perfect and just, so that they may be “witnesses over mankind” because of their justice and fairness, so they will pass judgement over the people of all other religions, but no one else will pass judgement over them. Whatever this ummah testifies is worthy of acceptance will be accepted, and whatever they testify is worthy of rejection will be rejected. 
If it is said: How can their ruling against others be accepted, because when there are two disputing parties, the word of one against the other cannot be accepted? 
The answer is that the word of one of the disputants cannot be accepted, if there is some doubt about his character. But if there is no doubt and there is certainty of justice and good character, as in the case of this ummah, then the aim is to rule with justice and truth. The condition of that is knowledge and fairness, which both exist in this ummah so its word will be accepted. End quote.