Paul was Peter’s brother in the faith (2 Peter 1:10) and also his colleague in service of the gospel (see also 2 Corinthians 1:1; 2 Corinthians 8:18, 2 Corinthians 8:22–23; Ephesians 6:21; Colossians 1:1; Colossians 4:7; 1 Thessalonians 3:2; Philemon 1:1; 1 Peter 5:12). Though Peter does not refer to Paul as an apostle, the term he uses can be understood as a reference to Paul being included among the other apostles.1 Alternatively it could simply single out Paul as being included amongst Christians in a general sense.2
The fact that Peter calls Paul our brother
rather than using (for instance) the term blessed Paul
is also a confirmation that Peter was the author of the letter.3
15 And count the patience of our Lord as salvation, just as our beloved brother Paul also wrote to you according to the wisdom given him,