Question: 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?

Answer: 
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](http://www.biblegateway.com/passage/?search=Matthew%2025:1-13) for just one example.
There isn't anything in the bible on Christmas lights, but in the Middle Ages, they used a candle as a sign of the star of Bethlehem. Originally candles were used, but in 1882 Edward Johnson invented the first string of electric Christmas lights. On a Christmas tree, the traditional topper is a star or a candle, then the lights around the tree symbolize a pathway to the star of Bethlehem where Jesus Christ was born.