Paul, Silvanus, and Timothy, to the assembly of the Thessalonians in God our Father, and the Lord Jesus Christ:
Silvanus ... is "the Latin form of Silas."[3] This was the man chosen by Paul following the dispute with Barnabas over taking John Mark on the second missionary tour (Acts 15:40), the reasons for such a choice probably lying in the good reputation Silas had (Acts 15:41), and the additional fact of his being a Roman citizen (Acts 16:48).  Paul's mention of him here, ahead of Timothy, was appropriate because of the share Silas had in the conversion of the Thessalonians; also, Silas had been chosen somewhat before Timothy was enlisted at Lystra.