J. S. Bach's last completed vocal work reveals the overwhelming wealth of his creativity and compositional skills.   Composed during Bach's time as cantor at the St. Thomas Church in Leipzig, it's heard here in the composer's domain as the crowning glory of the 2013 Leipzig Bach Festival.   Georg Christoph Biller leads the St. Thomas Boys Choir, an outstanding quintet of soloists, and the prestigious Freiburg Baroque Orchestra, playing on authentic period instruments.