What is the meaning of Allah's saying: (And thus we have made you a middle nation), and what is meant by bearing witness upon mankind?

The Ummah of Muhammad is a moderate, just, and chosen nation. On the Day of Resurrection, it will bear witness for the Messengers that they delivered the messages of their Lord, and the Messenger, peace and blessings be upon him, will bear witness for it. Its testimony is accepted due to its justice and the perfection of its religion, ethics, and deeds. What this Ummah testifies to as acceptable is accepted, and what it testifies to as rejected is rejected. This is solely due to its knowledge and justice.