The word 'believer' means a person who has faith. But how does a person become a believer? What is the process by which a believer comes into existence? From this perspective, a believer is one who has discovered reality. God Almighty is the greatest reality. Becoming a believer is the culmination of a process. 
A person is born with a mind. One's mind is a living machine which thinks constantly, in a non-stop manner. This thinking revolves around the questions: Who am I? Who is my Creator? What is the purpose of life? What is before death and what is after death? All these questions come to a believer's mind and they unleash a storm within. They make him go through brainstorming. After long study, observation and contemplation, he exclaims: Alhamdulillah Rabbil Alamin (Praise be to God, the Lord of the worlds!) He utters: "O God, I have conviction in You. I have brought faith in You!"
A person who undergoes this process is a believer. A believer is not merely a person who recites a certain kalimah as profession of faith. A believer comes into existence after experiencing a storm and upheaval in his thinking process. A person who becomes a believer in this way, develops all the qualities of true faith.