If a visitor is coming to your Sunday School, you can be sure that he or she already knows about Jesus and knows what Sunday Schools teach. So, the person has already been introduced to Jesus, and it would be discourteous to press the point. Just give him or her a warm welcome and listen attentively to what is said. Accept the diversity of beliefs in this world and perhaps afterwards you could express appreciation that a non-Christian has taken the time to visit your Sunday School.