Have Mercy Upon Me O God was written by William Byrd a 16th century composer of Renaissance music. He wrote various types of sacred and secular polyphony, keyboard, and consort music. He produced sacred music for use in Anglican services, although later, he became a Roman Catholic and wrote Catholic sacred music as well.
You will be transported by this lovely hymn performed sublimely by The Rose Consort of Viols, Tessa Bonner, Timothy Roberts & Red Byrd.
This is as good as it gets.