What is the passage mainly about?

Settlers originally came to the American continent so they could practice their religion as they wished, free from the religious persecution of that era. One of these religious groups that settled in the U.S. was the Shakers; (19)The Shakers prospered in the first half of the nineteenth century and, at their height, totaled about 6, 000 members. (18)They are, however, not known for their religion, but for the furniture they made during the first half of the nineteenth century. The furniture was solidly constructed, yet lightweight at the same time. The designs were also very simple because anything elaborate was not allowed, according to their religious principles. In the nineteenth century, their furniture was very popular because it was useful and durable. In the twentieth, people mostly value it for its beauty, in addition to its other characteristics. (20) Museums and collectors praise the furniture as one of the best examples of American design. Currently, only twelve aged Shakers remain, and none of them constructs furniture any longer. Hence, Shaker tables, chairs, cabinets, and other pieces are valuable collectors’ items because they are now very rare. Although the furniture may no longer be produced, the Shakers and their furniture will forever remain an important chapter in the art design of the United States.