The improviser’s art is music’s central magic. The repertory sustains itself around its power to state and then to vary; the organ is the supreme exerciser of this power. Dr. Hancock’s program would conclude, it was announced, with an “improvisation on submitted themes” as once did concerts by Mozart and Beethoven. The “submitted theme” this time was John Williams’ tune from Star Wars; one might have expected the worst. One would have been wrong; what we got instead was a beautifully fashioned, sophisticated, four-movement work that strayed far from the given theme, drew a splendid variety of thematic substances from its modest outline, ventured far into dark and complicated regions, and returned triumphant at the end. If this is what I’ve missed by ignoring church music for 62 years, perhaps it’s time to start listening.