Protestantism is a branch within the Christian denomination but is not associated with Catholicism, since it wanted to reform the Catholic Church during the 16th century. While it's perfectly fine to use both terms interchangeably, a better way of describing someone who follows Christianity would be 'a protestant follower of Jesus Christ since that more accurately describes what their faith entails. Protestants believe in salvation through grace alone, which is why some refer to them as being "reformed" sects rather than Christians.