God is faithful, by whom ye were called unto the fellowship of his Son Jesus Christ our Lord.
Faithful - to His promises (Philippians 1:6; 1 Thessalonians 5:24).
Called - according to His purpose (Romans 8:28).
Unto the fellowship of ... Jesus - to be fellow-heirs with Christ (Romans 8:17-30), being like Him, sons of God (2 Thessalonians 2:14; 1 Peter 4:13; 1 John 1:3). Chrysostom remarks, that the name of Christ is oftener mentioned in this than in any other letter, the apostle designing thereby to draw them away from their party admiration of particular teachers to Christ alone.