A believer or a true Christian is one who believes that Jesus is the Son of God, and that he fulfilled the Scriptures and the requirements of God’s law when he came and died for mankind.
John 20:31 “but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.”
In addition, a true Christian must give his life to follow Christ. He must renounce his earthly will and ambitions in order to fulfill the will of God, found in the Scriptures and thru the guidance of the Holy Spirit. A believer is not one who just believes (for Satan does that also) but he must continually seek to conform his life to the principles and standards found in the Bible, primarily the New Testament. See James 2:14-26 and Mat. 7:20.