Concise Bible Dictionary:
This word occurs but twice in the A. V.
1. πιστεύω, “to believe,” from πίστις, “faith” (Acts 5:1414And believers were the more added to the Lord, multitudes both of men and women.) (Acts 5:14)): “Believers were the more added to the Lord.” The same Greek word is constantly used for those that believe on the Lord Jesus Christ to the saving of their souls, as in John 3:15-1815That whosoever believeth in him should not perish, but have eternal life. 16For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. 17For God sent not his Son into the world to condemn the world; but that the world through him might be saved. 18He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God. (John 3:15‑18).
2. πιστός, “trusting, trusty,”‘ also from πίστις, “faith” (1 Tim. 4:1212Let no man despise thy youth; but be thou an example of the believers, in word, in conversation, in charity, in spirit, in faith, in purity. (1 Timothy 4:12)); “be thou an example of the believers.” The same word is used for “What part hath he that believeth with an infidel, or unbeliever?” (2 Cor. 6:1515And what concord hath Christ with Belial? or what part hath he that believeth with an infidel? (2 Corinthians 6:15)). “Believers” are a class of persons, who, through the mercy of God, have faith in Christ as the Son of God, and in His atoning work on the cross. See FAITH.