From BiblePortal Wikipedia

Charles Buck Theological Dictionary [1]

An appellation given, toward the close of the first century, to those Christians who had been admitted into the church by baptism, and instructed in all the mysteries of religion. They were thus called in contradistinction to the catechumens who had not been baptized, and were debarred from those privileges. Among us it is often used synonymously with Christian.

See Christian

International Standard Bible Encyclopedia [2]

bē̇ - lēv´ẽrs (in the King James Version and the Revised Version (British and American) of   Acts 5:14 , for ποστεύοντες , posteúontes , the Revised Version, margin "believing"; in the King James Version of  1 Timothy 4:12 for οἱ πιστοί , hoi pistoı́ , the Revised Version (British and American) "them that believe"): Equivalent phrases, they (he, she) that believe (for οἱ πεπιστευκότες , hoi pepisteukótes  ; οἱ πιστεύοντες , hoi pisteúontes  ; (adj.), πιστός , pistós , etc.) occur frequently as a regular description of those who professed their faith in Christ, and attached themselves to the Christian church. The one essential condition of admission into the Christian community was, that men should believe in Jesus Christ ( Acts 16:31 ). The actual experiences of the men Thus denoted varied with all the possible degrees and modifications of Faith (which see). Believers are nowhere in the New Testament distinguished as a subordinate class from the "Christians who know" as in the Gnostic antithesis of pistikoı́ and gnōstikoı́ , "believers" and "knowers."