I visited the Trinity Cathedral inside the monastery after reviewing the history of the area with my guide. Built in 1422-23 over the grave of St. Sergius, this is widely considered to be the most important structure in the monastery. Even at this point in my travels I had come down a bit with some "church fatigue" as they all seemed to start having more similarities than differences. What made this experience different was the active mass going on as we entered the cathedral; it was easily one of the most memorable moments of my ten days in Russia. Whereas the churches in the Kremlin seemed to function as religious museums, the Trinity Cathedral came to life as I watched about fifteen minutes of a Russian Orthodox service and lit candles before departing. For the first time I really got the sense of the importance of churches such as this in centuries past. Highly recommended.