“So we beat on, boats against the currents, borne back ceaselessly into the past.” Nick Carraway’s concluding insight in The Great Gatsby is one of the great closing sentences in literature, and one of the great images of our human helplessness to escape the past. It’s also the line that ends John Harbison’s Gatsby opera, which—13 years after its premiere at the Metropolitan Opera—just had its first complete Boston performance, in a concert version produced by Emmanuel Music (the musical organization Harbison co-founded in 1970 with Craig Smith at Boston’s Emmanuel Church, mainly to play all of Bach’s cantatas as part of every Sunday’s liturgy). Harbison is now Principal Guest Conductor at Emmanuel, which has long been associated with his music, including the very first public performance, in 1997, of the first two scenes from The Great Gatsby.