How do we know who will stay true until the end?  The answer is that we don't!  Luke did and Demas did not.  Luke, the author of Luke's Gospel and the book of Acts, had travelled with Paul since joining him at Troas on the second missionary journey (indicated by the change from 'they' to 'we' in Acts 16:11).  At the time of writing, Luke was still with Paul and remained committed to the task of completing the Book of Acts.  

However, Demas seemed equally committed.  He was one of the believers who was allowed to provide for Paul in prison – trusted by Paul and a well-respected man by the believers in Rome and in Colossae.  But towards the end of Paul's life, with the possibility of being thrown to the wild animals in the arena, he writes, "... for Demas, because he loved this world, has deserted me and has gone to Thessalonica ..." (2 Timothy 4:10).  Demas knew the gospel was true and yet his deepest love was not for Jesus and His people, but for the apparent safety that he could create in this world.
 
And what about the churches in Colossae, Laodicea and Hierapolis?  The real local church is not a building but the group of believers who meet together where they live.  When they move because of persecution, war, famine, lack of work or death, that congregation comes to an end, unless they are replaced by others.  We know that God used some willing individuals to serve Him - like Nympha who welcomed believers to meet in her house.  It is the individuals who need to stand firm to the end wherever they live or move to.  Each believer needs to continue in Christ (Colossians 1:22-23).  But the church congregations in Laodicea and Colossae did not continue.  Perhaps the clues to the demise of the church at Laodicea are in Revelation 3:14-22 - they became self-satisfied and kept Jesus outside the church.
 
The application should be obvious.  However committed we appear to others, we can fall away from Jesus Christ if we allow our own interests to dictate our lives, and leave the Lord out of the centre of our desires (Romans 11:22).  Like the Laodicean church, it can happen gradually: or like Demas it can be the final decision of a reluctant heart (unwilling to sacrifice everything for Jesus).  The key is to practise giving up your 'rights' - like Nympha did in opening her home to others. It may be giving up possessions, money, status, power, or some of the pleasures of life (Acts 2:44-47).  Once that happens, because of obedience to Jesus, then it becomes normal to trust Him with all the uncertainties of the future.  People, who live with Him in control, get to the end in good order (2 Timothy 4:6-8): but those who live by clinging onto control risk being found to be unfaithful at the end (Matthew 7:21-23).  Which are you?