Believer is a word originating from the Latin term credens which means who believes. A believer is that individual who believes in something.
Popularly, it is said that a person is a believer who believes in a naive or mistaken way in something, creating some expectation, for example: "She believes that she will pass the exam." Or, the credulous and naive individual, who believes in everything that they say.
In the religious context, a believer is anyone who believes in God and expresses his or her religious belief.
Religious belief is related to the dogma that Jesus Christ, the Son of God, is the only way to soul salvation and redemption from sin.
Evangelicals, members of certain evangelical churches (religious groups that strictly follow the Gospels), are also particularly called believers.
Evangelicals are often called Protestants too. However, not all evangelical denominations originated from the Protestant Reformation led by Martin Luther.
Those who follow the doctrines of Christianity and live exemplarily according to the teachings of Christ are called Christians.