His teaching is: generosity of spirit. Do not just do what is required; go above and beyond what is required! Generously sacrifice of yourself and give, even though it is unfair. Everybody seems to agree that what He was talking about here was a common practice. That is, the mail was generally carried by the army (in Roman times). The soldier would get tired of carrying the mail. So he would get some civilian and say, "Here. Carry this." And he [the civilian] would have to carry the mail, while the soldier walked along beside him—unburdened by the weight of the mail. So, Jesus said, "Go two miles." That is, go above and beyond. The point is: generosity of spirit—even in terms of sacrifice.
