In 2 Corinthians 7:3, Paul repeats that the Corinthians are in his heart — the same affectionate phrase from 3:2.

In 1 Corinthians 9:1, Paul directly calls the Corinthians his workmanship in the Lord — the same idea as them being his letter of recommendation.

In 1 Corinthians 9:2, Paul calls them the seal of his apostleship — a direct parallel to them being his living letter.

Acts 18:27 shows written letters of recommendation — the very custom Paul references when he calls the Corinthians his living letter.

In Philippians 1:7, Paul holds the Philippians in his heart — similar to the Corinthians being written on his heart, showing his pastoral affection.