‘Luke, the beloved physician, and Demas salute you.'
Luke was a regular companion of Paul (note the ‘we' passages in Acts - see Acts 16:10-16; Acts 20:6 to Acts 21:18 and probably after; Acts 27:1 to Acts 28:31), and the author of Luke and Acts. It is here that we learn he was a physician. In Philemon 1:24 he is a ‘fellow-worker'. This demonstrates that he did more than just being a doctor. He was also active in Christian service and ministry.
‘And Demas.' Ranked with Aristarchus and Luke as ‘fellow-workers' in Philemon 1:24. Later he would fear for his life and leave Paul (2 Timothy 4:10).