What is the religious significance of the modern practice to decorate the outside of the house and front yard with strings of light? Is it entirely secular?
Well, there certainly isn't anything in the Bible about it, for the simple reason that there isn't anything in the Bible about celebrating Christmas in the first place. (Not to mention electricity!) However, hanging or holding up lanterns to provide festive illumination after dark for festivals or celebrations is an ancient custom in many cultures, dating back to Old Testament days. See the the Parable of the Ten Virgins for just one example.