Francis Schaeffer used a phrase "the back of the book" to talk about man's nature. Because every single person is made by God, there are certain things that are true for all people. As Christians we know those things and can make wise use of them when talking to a non-Christian. One of these facts is what Greg calls "moral motions." Every person is a moral being and J. Budziszewski is the expert on natural law - what we can't not know. Certain moral principles are so basic that we can't not know them and every person does know them and that fact can help in persuading someone of the truth of the Gospel. Budziszewski says, "The object isn't for you to show that you see through his self-deceptions. It's for him to begin seeing through them."
A wise ambassador can help someone recognize the truth God has already planted in his heart that is self-evident. It isn't always easy because man has a great capacity for self-deception, but we can patiently help someone who is seeking the truth to shed that deception and see the truth.
You can read an interview with J. Budziszewski in Australian Presbyterian by following the link at Between Two Worlds. As a bonus, there's a good article by Dr. Scott Rae on business ethics that builds on the same principle.