It's hard for even secularists to not love Bach's St. Matthew Passion. Still, the work's double orchestra, multi-part choral arrangements (auf Deutsch, natürlich), and three-hour running time are a bit much to digest on first listen. To help sort out the complexities of the classic piece — which sets the Gospel of St. Matthew, chapters 26 and 27, to music — 98.7 WFMT's Carl Grapentine appears tonight for a free, pre-concert lecture on the finer points of the composer's work. Afterwards, head over to the Harris Theater for Chicago-based Music of the Baroque's 7:30pm performance of the Matthäuspassion.