Beethoven’s Missa Solemnis is indisputably one of the great milestones of Western civilization, a fervid profession of faith expressed in the classical style at its most mature. It is mandatory listening for those who care about musical humanism. This presentation, postponed from two years ago due to the musicians’ strike, is only the seventh in the CSO’s history. It comes at a time when music director Daniel Barenboim seems to be under siege, unable to match Georg Solti’s success. The 50-year-old maestro’s touted versatility, unfortunately, is also his main weakness; he spreads himself so thin, over a repertoire that spans centuries and genres, that he’s almost a master in none (except, perhaps, as a piano recitalist). Though not yet a first-rate Beethoven interpreter–despite his Furtwangler wannabe tendencies–he can go a long way toward quieting the Solti loyalists by turning in a memorable performance here. One thing in his favor is that he’ll be working with a quartet of his favorite singers: soprano Tina Kiberg, mezzo Waltraud Meier, tenor John Aler, and bass Robert Holl. May promises to be a busy month for Barenboim: his varied activities will also include guiding the CSO in a pair of world premieres (featuring pieces by Shulamit Ran and German composer York, Holler), directing the Civic Orchestra, and partnering Itzhak Perlman in a traversal of the entire Beethoven violin sonata cycle. This performance, however, may well be the defining moment of his CSO tenure. It would be churlish not to wish him luck. Friday and Saturday, 8 PM, and Tuesday, 7.30 PM, Orchestra Hall, 220 S. Michigan; 435-6666 or 435-8122.