God is faithful, by whom you were called unto the fellowship of his Son Jesus Christ our Lord.
All Commentaries on 1 Corinthians 1:9 Go To 1 Corinthians 1
Ambrosiaster
AD 400
Fellowship is brotherhood. Just as Paul declares God’s unfailing faithfulness toward us in this regard, so we ourselves must not be found to be faithless or dishonorable with respect to our adoption. Rather we must remain faithful in it. Commentary on Paul’s Epistles.