Chronologically, Philemon comes before 2 Timothy, though it appears later in the New Testament. That’s important to know when looking at references to Demas.
We first read of him as Paul closes his epistle to the church at Colosse.
Luke, the beloved physician, and Demas, greet you. (Colossians 4:14 KJV)
Next in the timeline is in the letter Onesimus carried from Rome to Philemon.
There salute thee Epaphras, my fellowprisoner in Christ Jesus; Marcus, Aristarchus, Demas, Lucas, my fellowlabourers. (Philemon 1:23-24 KJV)
We do hear of him a bit later, when Paul writes again to Timothy.
For Demas hath forsaken me, having loved this present world, and is departed unto Thessalonica; Crescens to Galatia, Titus unto Dalmatia. Only Luke is with me. Take Mark, and bring him with thee: for he is profitable to me for the ministry. (2 Timothy 4:10-11 KJV)
The Greek word used here for “loved” is agapao – derived from agape, the spiritual love between God and man. Instead of loving God, Demas left Paul for instant gratification. We know nothing more of his story, though speculation continues. Many would make excuses for Demas, trying to explain that this happens often in life.
Paul said nothing more about Demas, no condemnation, no recriminations, no comments regarding his future and no pleas for his return. Paul states facts, not judgments.
James puts a bit more perspective for those who chose the world – and he doesn’t define when this choice takes place, before or after committing one’s life to Christ.
Ye adulterers and adulteresses, know ye not that the friendship of the world is enmity with God? whosoever therefore will be a friend of the world is the enemy of God. (James 4:4 KJV)
Paul also wrote of the sorrow of the world.
For godly sorrow worketh repentance to salvation not to be repented of: but the sorrow of the world worketh death. (2 Corinthians 7:10 KJV)
We know nothing more of Demas. But, we are faced with the same choices – follow Christ or enjoy the pleasures this world has to offer without Him. Some have chosen well, some have forsaken their faith for things important only in this world.
Of course, if a person believes this is the only life there is, why not? I’ve chosen to believe there is more to life than this present world. My doing so harms no one, even when I state there are consequences for not believing so. Others do not believe.
If the Bible is wrong, we will never know. If the Bible is God’s word, our choices determine our eternal results. The stakes are very high and the question deserves careful consideration.