Silas is a Jewish Christian who appears to have been active in the Jerusalem church, assuming that the Silas mentioned in 15:22 is the same man (See Witherington, Acts, 473.). That Luke should mention a character in one context then pick him up again later is a common feature of the book.
Silas is likely an Aramaic form of Saul. The Silvanus of 1 Peter 5:12 is likely the same man since Peter would have know Silas from Jerusalem. Silas is mentioned frequently in Paul’s letters, 2 Cor 1:19, and he is a “co-sender of the Thessalonian letters (1 Thess 1:1 and 2 Thess 1:1). It is therefore suggested that he functioned as a secretary for Paul in the writing of these letters (and perhaps others). 1 Thess 2:6 refers to the “apostles of Christ,” which may imply that he was considered an apostle like Barnabas, although not from the Twelve.
Silas was a Roman citizen since Acts 16:37 implies that he was imprisoned illegally. This would seem to imply he was a Hellenistic Jew. His name confirms his: he is also known as Silvanus, a Roman cognomen meaning “wood,” and the same name as the Roman god Silvanus, a life-giving deity (Gillman, “Silas,” in ABD 6:22).
He travels with Paul for most of the second missionary journey. In Acts 17 he and Timothy travel back to Berea and Thessalonica while Paul travels to Athens and then to Corinth. R. C. Campbell suggests that since Silas was able to return to these locations indicates that Silas was “less controversial” than Paul (ISBE Rev, 4:509). This might be true, but it may be that Silas was more acceptable to the Jews socially and theologically than Paul.
So why Silas? Like Barnabas, he was a Hellenistic Jew yet he was firmly rooted in the Jerusalem church. Paul seems to have wanted a companion who was “acceptable” to Jerusalem, perhaps to preempt any criticism of his Gentile mission by the more conservative elements of the Jerusalem church. Paul would therefore represent the Antioch churches, Silas the Jerusalem churches, implying that any mission to the Gentiles was co-sponsored by both centers of Christianity.