This is the law of nature the golden rule of all justice, and may also serve for a guide to us to expound the former verses, and some other precepts of charity in this chapter. Men in all these cases should consider what they would be glad, and think reasonable, that others should do to them, were they in their circumstances, and the others had the same ability or advantage to do good to them; and by this they should measure their acts both of justice and charity.
Christ came to teach us, not only what we are to know and believe, but what we are to do; not only toward God, but toward men; not only toward those of our party and persuasion, but toward men in general, all with whom we have to do. We must do that to our neighbour which we ourselves acknowledge to be fit and reasonable. We must, in our dealings with men, suppose ourselves in the same case and circumstances with those we have to do with, and act accordingly. There are but two ways right and wrong, good and evil; the way to heaven and the way to hell; in the one or other of these all are walking: there is no middle place hereafter, no middle way now. All the children of men are saints or sinners, godly or ungodly.
[Excerpt from Matthew Poole's Commentary & Matthew Henry's Concise Commentary]