Demas once walked side by side with the Apostle Paul. But he became entangled in the cares of this world (as Jesus warned would happen in the parable of the sower) and departed from the faith. John spoke of this in his first letter, 1 John 2:19 where he clarifies that the reason why those leave is because they were never really there to begin with.
John warns that there is a path that leads to this falling away:
“Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him. For all that is in the world—the desires of the flesh and the desires of the eyes and pride of life—is not from the Father but is from the world. And the world is passing away along with its desires, but whoever does the will of God abides forever.”
1 John 2:15-17 ESV
Everyone professing the name of Christ WILL BE TESTED. Our motives will then be revealed. If the foundation of our faith is built upon “what He can do for me in this life”, our foundation will fall and along with it our faith. Like Demas, the root desire of our heart will ultimately prevail.
Paul warns that we’re to check our motives, to check to see if we are truly in the faith. Because in this we have opportunity to repent and get our eyes off the world and back on Christ.