William Barclay, the great Scottish commentator, suggests that there may be some reason to believe, however, that Demas later returned to fellowship with the Lord. He points out that the name Demas is a nickname for Demetrius, and that there are two Demetriuses mentioned in Scripture. One was the silversmith in Ephesus who led the riot, recorded in the eighteenth chapter of Acts, that threatened the apostle’s life. There is at least a slight suggestion that he may have been converted because of that encounter and became the Demas who accompanied Paul on some of his journeys. Paul refers to him in the letter to Philippians as, “my fellow laborer.” Now that his love has cooled, however, Demas has returned to Thessalonica, where he perhaps originally came from, turning away, at least, from a forthright Christian testimony.
But in the letters of John, who wrote somewhat later than Paul, another Demetrius is mentioned. This man seems to be a very responsible, dependable Christian. Barclay suggests that this, perhaps, is the whole story of Demas; that he began as a pagan and an idol worshipper, was converted, accompanied the apostle, then drifted back into the world for a while, but eventually God reached him again and turned him around, and he ended his life as a respected believer.
This highly speculative, however. There is very little evidence for that. As far as we know, this is the last picture we have of Demas. He stands for those heartbreaking cases who have begun well, who seem to hold forth much promise of faithfulness in Christ, but drift back again into the empty pleasures and commerce of the world, lose their Christian testimony, and nothing further is heard of them. This was a great heartbreak to the apostle; he writes these words with a tremendous sense of loss. (Taken from a sermon on II Timothy 4:9-22 by Ray Stedman)