The term "salvation" in Scripture is spoken of in past, present, and future terms.
Past: We have been saved from the penalty of sin--> "justified" (declared righteous because of Christ's perfect obedience, satisfaction on the cross, and resurrection); God regenerated us and gave us a new nature, and He broke the chains of sin and thus set us apart for obedience (definitive sanctification).
Present: We are being saved from the power of sin--> "sanctified" (growing in holiness and in the image of Christ). This is called "progressive sanctification."
Future: We will be saved from the presence of sin--> "glorification."
The Bible equally teaches that to be "glorified" we must persevere in the faith until the very end of our lives.
But the term "perseverance" has been misunderstood as well. We are to strive in our sanctification and our perseverance. We may not remain complacent or lazy. We must progress, and we must continue progressing. If we do not, we will not gain heaven.
The Scriptures are clear on this. We are warned several times in Hebrews to strive to persevere. Hebrews 4 says, "Since we have the promise of entering His rest, let us make every effort to enter that rest."
Paul tells us to run in such a way that we may get the prize in 1 Corinthians 9. "I beat my body and make it my slave, lest I be disqualified for the prize."
Hebrews 12 tells us to strive for the holiness without which no one will see the Lord.
Jesus tells us to make every effort to enter through the narrow gate. He says that many will try, and will not be able.
He also warns us to not be weighed down by the cares of this life. He warns His disciples to watch and pray, that they may be able to stand before the Son of Man.
Jesus says that the kingdom of God, since the days of John the Baptist, has "suffered violence, and violent men take it by force." Have you considered what our Lord means here? He is saying that we will not persevere in the faith and lay hold of heaven without violence and mortification in our sanctification.
Paul the apostle warns us not to be arrogant, but to fear in Romans 11.
Paul also considers himself not to have taken hold of it yet in Philippians 3.
All this is to say that, even though on the one side the Bible speaks of election and the assurance of perseverance, and that the elect can never lose their salvation, on the other hand it exhorts us to make our calling and election sure (2 Peter 1). We are to make sure that we really are of the elect. There have been many who "knew" they were elect, and abandoned the faith.
We therefore must watch and pray, and "wrestle with God" for our perseverance.
We need to have a healthy fear against falling away.
The Scriptures are replete with passages on this issue.
I think three books which look at this in an excellent biblical light are the following:
The Heavenly Footman, by John Bunyan.
The Pilgrim's Progress, by John Bunyan. (Note especially the warning that Interpreter gives to Christian when Christian observes the Man in the Iron Cage.)
Heaven Taken By Storm, by Thomas Watson.
May the Lord grant that we would not be slothful, but that we would press on in our faith and lay hold of Christ, our Prize.