This is a good question. Demas is an obscure figure in the New Testament. He is only mentioned three times. He is mentioned in Colossians 4:14, Philemon 1:24, and 2 Timothy 4:10.
In Colossians 4:14, he is a companion of Luke and he sends greetings to the church at Colossae. In Philemon, he is noted as a fellow worker with Luke. It appears that Demas has joined Luke in the work of ministry. To what extend, we can only guess, as the text is not clear. However, he seems to be progressing in his faith. At first he is a companion and now a laborer.
However, he is mentioned again in 2 Timothy 4:10, but something has change. 2 Timothy 4:10 states, "…Demas, because he loved this world, has deserted me and has gone to Thessalonica."
In light of this point, some interpreters warn people of the dangers of the world.