How do we respond to a Christian who interprets the verse: "You were the best of nations brought forth for mankind, enjoining what is right and forbidding what is wrong" by saying that the Islamic nation was the best nation in the past and that "were" (كان) here is a past tense verb indicating cessation?

The meaning of "kāna" (was/is) is not limited to past tense and cessation; rather, it carries other meanings such as permanence and continuity, or it can mean the present state. The Muslim Ummah, described as the best nation, is that which fulfills the condition of enjoining good, forbidding evil, and believing in Allah. For this reason, some exegetes have specified this superiority to the early generations (Salaf) of the Ummah. Despite the shortcomings of the Muslim Ummah, it remains the best of nations because others have even greater shortcomings. Religious texts have affirmed that the Ummah of Muhammad (peace be upon him) is the best and most honored of nations in the sight of Allah, and it has gained precedence due to its Prophet Muhammad (peace be upon him). Whoever undertakes to debate with Christians must possess sufficient knowledge to refute misconceptions and clarify the truth; otherwise, they should avoid it.