Calvinism is a Reformed tradition of Christianity that originated with John Calvin during the Protestant Reformation in the 1530s. Though notable for a number of tenets such as predestination, total depravity, unconditional selection, limited atonement, irresistible grace, and perseverance of the saints, the reference here relates to Calvinism’s rejection of any practice of worship not explicitly approved in the New Testament.
Thus, driven by antipathy toward many of the religious practices of Roman Catholicism, Calvinists rejected musical instruments and much verbal content in the services of other sects. Many churches across Europe were vandalized, and emptied of their art and valuables, because of acquisitive greed, or hatred for the Catholic Church, or taken over by Calvinist congregations who simplified the decor for theological reasons (which is what the narrator of Unbearable Lightness is talking about).
The line in Shakespeare’s famous Sonnet 73 about old age, “Bare ruined choirs, where late the sweet birds sang,” may refer to the many Catholic churches abandoned and vandalized in England under Queen Elizabeth, after Henry VIII broke away from the Roman Catholic Church.