The preacher was contrasting Demas and Barnabas. Demas is only mentioned three times in the New Testament: here are those three instances in chronological (but not textual) order:
Epaphras, my fellow prisoner in Christ Jesus, sends you greetings. And so do Mark, Aristarchus, Demas and Luke, my fellow workers.Philemon 1:23-24
Our dear friend Luke, the doctor, and Demas send greetings.Colossians 4:14
Do your best to come to me quickly, 10for Demas, because he loved this world, has deserted me and has gone to Thessalonica.2 Timothy 4:9-10a
The preacher pointed out how Demas was at one time a fellow-worker, a "good guy", but that near the end of Paul's life had deserted Christ because he "loved this world". He then went on to preach that Demas had become a child of Satan (I don't recall if those were the preacher's words, but it's the meaning I got from his sermon).
It seemed very harsh and judgmental to me.
Perhaps my thinking capability is faulty, but it seems to me that the preacher's conclusion is not warranted, especially when you look a bit closer at the context.
As part of his comparison of Demas with Barnabas, he pointed out Acts 15 which talks about Paul and Barnabas wanting to revisit the churches they had earlier established. On their earlier trip they had taken John Mark, who deserted them part way through the trip. Now Barnabas is wanting to take John Mark again, but Paul won't have anything to do with John Mark, and the result is that Paul and Barnabas go their separate ways.
The preacher was focusing on Barnabas and how he was an encourager (Barnabas means "son of encouragement"), as opposed to Demas who had "become a son of Satan". But he missed the detail about Paul's bitterness at being abandoned. Obviously Paul doesn't cotton to folks who may have good intentions but bail on him for whatever reason.
We don't know why John Mark deserted them on their first missionary trip. Nor do we know why Demas deserted Paul, other than that he "loved this world". We do know that Paul has already demonstrated a bitterness about being deserted.
The preacher equated this loving of the world that Demas was guilty of with the love of the world condemned in 1 John 2:15 -- "If anyone loves the world, the love of the Father is not in him." I'm not entirely confident that the two similar phrases mean the same thing in both contexts. (Nor am I convinced they don't mean the same thing -- that's my point: the evidence is not clear one way or the other, so don't just choose one way and then turn that into "Gospel Truth".)
When we read further in 2 Timothy 4, we find that Demas is not the only one that deserted Paul:
Do your best to come to me quickly, 10for Demas, because he loved this world, has deserted me and has gone to Thessalonica. Crescens has gone to Galatia, and Titus to Dalmatia. 11Only Luke is with me. Get Mark and bring him with you, because he is helpful to me in my ministry. 12I sent Tychicus to Ephesus. 13When you come, bring the cloak that I left with Carpus at Troas, and my scrolls, especially the parchments.Paul is in prison; all his friends have abandoned him for one reason or another. He has enemies. It's not surprising that he might feel some bitterness, and that's consistent with his behavior many years earlier in conjunction with John Mark.
14Alexander the metalworker did me a great deal of harm. The Lord will repay him for what he has done. 15You too should be on your guard against him, because he strongly opposed our message.
16At my first defense, no one came to my support, but everyone deserted me. May it not be held against them.
It seems to me that Demas left, like everyone else, when the heat got too great against those who supported Paul (v. 16a). But Paul, realizing that he was letting his anger get the better of him in his letter to Timothy, then asserted his discipleship of Jesus by forgiving those deserters -- "May it not be held against them."
Maybe Demas really does belong to a different class of deserters than do these others that Paul is forgiving; after all, Demas is the only one specifically mentioned as having loved the world. But it seems to me that all the deserters loved the world, trying to save their own skins by "gettin' while the gettin's good". Perhaps Paul was more bitter toward him than toward the others, since they had been so close to each other. I don't want to excuse Demas' failure to be faithful, but neither do I want to condemn him without warrant. As far as I know, he's just like Peter, who denied the Lord three times at the Lord's trial, after swearing just a few hours earlier that he'd never do such a thing. Peter repented. Perhaps Demas did also. The point is, we just don't know.
And to claim that we do know that Demas lost his salvation based on this flimsy evidence is, to my mind, going beyond the text.
I think much of the problem is in the equating of Demas' love of the world with the love of the world condemned in 1 John 2:15. But just because the phraseology is similar, does not necessarily mean the meaning is similar. After all, there's another place with similar phrasing -- John 3:16 -- "For God so loved the world ...". (I'm not implying that Demas loved the world the same way God did; that's obviously not the case. But my point is that similar phraseology can mean different things in different contexts.)
Demas may have indeed turned his back on Jesus, but without better evidence, I'm withholding judgment against him, choosing to believe the best of him (1 Cor. 13:7 AMP), and choosing to hope that like Peter, he returned into the fold of the Master, and like the other deserters, that he was included in Paul's prayer that desertion would not be held against them.