Letters on the South (Left) Wall of the Nave from Lk 22:32:
"I have prayed for you Peter, that your faith may never fail; and you in turn must strengthen your brothers"

Letters on the North (Right) Wall of the Nave from Mt. 16:19
"I will entrust to you the keys of the kingdom of heaven. Whatever you declare bound on earth shall be bound in heaven; and whatever you declare loosed on earth shall be loosed in heaven" http://stpetersbasilica.info/Interior/Nave/Nave.htm