One of the most powerful pieces of music I know is Giuseppe Verdi’s setting of the Roman Catholic requiem mass.
It would have been especially powerful, I expect, to have heard Verdi’s Requiem performed by the prisoners in the Nazi concentration camp at Theresienstadt under the direction of Rafael Schächter, who was trying, symbolically, to communicate the circumstances at the camp to a Red Cross delegation that visited Theresienstadt in 1944. I wonder whether the members of the delegation grasped the message. I hope so. One of the questions that occupied the minds of the inmates of the camps was whether anybody in the outside world knew and understood what was happening.
You can watch a great performance of the Requiem here.