Mentioning Demas reminded Paul of more faithful coworkers Luke had traveled much with Paul, writing both the Gospel of Luke and the book of Acts. (much of what is a first hand account) Tychicus, one of his most trusted companions (Acts 20:4:, Eph.6:21;, Col. 4:7; Titus 3:12) had already left for Ephesus.
[so then, what do we learn?] There are two ways to love the world God loves the world as He created it and as it could be if it were rescued from evil; others, like Demas, love the world as it is, sin and all. Do you love the world as it could be if justice were done, the hungry were fed, and people loved one another? Or do you love what the world has to offer– wealth, power, pleasure- even if gaining it means hurting people and neglecting the work God has given you to do?
As we work through each day with challenges, questions and or frustrations, we can stay in prayer and stay in faith because we know God is in control and our trust is in Him.