Albert Barnes on 1 Thessalonians 1:1

Paul, and Silvanus, and Timotheus. On the reasons why Paul associated other names with his in his epistles, 1Cor 1:1, 2Cor 1:1. Silvanus, or Silas, and Timothy were properly united with him on this occasion, because they had been with him when the church was founded there, Acts 17, and because Timothy had been sent by the apostle to visit them after he had himself been driven away, 1Thes 3:1,2. Silas is first mentioned in the New Testament as one who was sent by the church at Jerusalem with Paul to Antioch, Acts 15:22; and he afterwards became his travelling companion.

Which is in God the Fathers and in the Lord Jesus Christ. Who are united to the true God and to the Redeemer; or who sustain an intimate relation to the Father and the Lord Jesus. This is strong language, denoting that they were a true church. Comp. 1Jn 5:20.

Grace be unto you, etc. Rom 1:7.

(a) "and Timotheus" 1Pet 5:12 (b) "the Thessalonians" Acts 17:1 (c) "Grace" Eph 1:2