The thought that begins here is not completed until verse 6, which tells us that what is impossible is that we should get as far as the state described in verses 4 and 5, and yet we should fall away and then expect to come to genuine repentance after that. There is a solemn warning here.
Strangely, the doctrine of the perseverance of the faith is the most contested of all the so-called points of Calvinism. Maybe that is because of the existence, in different places at different times, of many nominal Christians. We believe that the great confessions of the 17th-century were quite correct in affirming the preservation of believers by the power of God after conversion. It is not for nothing that the doctrine was called, not the preservation of the saints, but the perseverance of the saints. The teaching is that once a person is truly converted by the help and the blessing of God, they will persevere to the end. They will be people who strive against sin and for holiness. If they fall, they are disturbed by this, even if their fall is profound and God has to discipline them. They will be brought back to the place of repentance and they will be seen back on the road of holiness and striving after heaven, after the Lord. The perseverance of the saints teaches that those who are truly converted will not simply be kept automatically, regardless of their lives. They will be caused to persevere and strive to please Almighty God and Christ their Saviour. That's the doctrine. It is not only a promise of God, but it is as a sign of true salvation. If you see somebody who does not persevere, somebody who after a profession of faith falls into complete indifference and waywardness, and stops the exercise of faith, not just for a time as a temporary backslider, but in an ongoing way and a settled way, then you conclude that person could not possibly have been converted in the first place.