Who were the Puritans? What were some of their beliefs? What did they think of the theatre and the actors in Shakespeare's time?

The Puritans were radical Protestants. They were extraordinarily devout, dedicated to doing away with the pomp and ceremony of the Roman Catholic Church.
They believed that worship should be simple and regular and that most forms of luxury and pleasure were inherently sinful and should be avoided at all costs.
They hate the actors and owl the theatre because they were forms of entertainment. They think that they served as a graphic illustration of the moral decline of the country. Many Puritans called for a complete ban on such activities.