The major difference between Roman Catholics and Protestants is the place given to the Bible. Although Roman Catholics believe that the holy Scriptures are inspired of God, they also believe that, along with the Bible, the tradition of the church through history, brought about by church councils, is to be given weight in matters of faith and doctrine. Protestants believe that the sole rule of faith and practice should be the holy Scriptures, and therefore, they do not accept certain Catholic practices that are primarily justified by appealing to tradition. They do not accept the authority of the pope. They do not accept the practice of praying to the Virgin Mary, and they do not accept other rituals practiced in Roman Catholicism.
At the time of the Reformation, a Roman Catholic monk named Martin Luther rediscovered the doctrine of justification by faith set out by the apostle Paul in the book of Romans (see Romans 3:24-30). If a man is justified from his sin by faith alone, Luther reasoned, then what is the need for pilgrimages, penance, indulgences, purgatory and the last rites? Since Luther's findings were interpreted to undercut Roman Catholic tradition, he was excommunicated. However, large numbers of Germans flocked to hear him, and there arose what came to be the Lutheran church.
Then, other theologians such as John Calvin challenged Roman Catholic teaching, and Calvinistic churches known as Reformed churches emerged. These churches, arising out of a "protest" to reform Roman Catholicism, were called Protestant churches. That period of history, approximately from 1500 to 1650, is called the Protestant Reformation.