God is faithful, by whom you were called into the fellowship of his Son, Jesus Christ our Lord.

1 Corinthians 1:9 (ESV)

This is ‘the fellowship.’ It belongs to God’s Son, who purchased it with his blood, and is distinct from the precious truth of ‘fellowship with’ in 1 John 1:3.
The whole phrase makes it clear. They call upon Jesus as Lord, thus committed to observing all that he and his apostles commanded. God is faithful. Are we?

Daily Reading: Acts 10:1-23