How Do I Know I Will Persevere? - by William Lane Craig

Excerpted from here. Some food for thought concerning the idea of "once saved, always saved."
---------------

I think that the New Testament teaches that regenerate Christians can fall away and lose their salvation. So I don’t agree... that Christians who for some reason or another fall away from the faith “were never saved in the first place.” On the contrary, I think that they were saved but by apostatizing have forfeited salvation. Just think about the warning given by the author of Hebrews against apostasy:

For it is impossible to restore again to repentance those who have once been enlightened, who have tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, if they then commit apostasy, since they crucify the Son of God on their own account and hold him up to contempt. (Hebrews 6.4-6)

It seems clear that he is describing here a genuine, regenerate Christian, not some unregenerate, counterfeit Christian. The danger of apostasy is very real and needs to be taken seriously by every true Christian.

Now this doctrine might seem at first blush to rob us of our assurance of salvation. But the opposite is true. I recall my theology professor Clark Pinnock once remarking that if Hebrews 6.4-6 could be a description of an unregenerate person, then what assurance can any of us have of being regenerate? As your letter so poignantly illustrates, the person who thinks that such a description could fit an unregenerate person on his way to hell may be hag-ridden with doubt that he is truly saved, despite his living and confident faith. By contrast, you can be assured that you are saved on the basis of the witness of the Holy Spirit with your spirit that you are a child of God (Romans 8.15-17).

On the same basis, you can be confident that you will never “discover some argument/fact that destroys [your] faith.” Even if you encounter objections or defeaters of your Christian belief that you cannot yourself in turn defeat, the witness of the Holy Spirit is an intrinsic defeater-defeater that simply overwhelms the defeaters brought against Christian belief by exceeding them in warrant.

Thus, no one will apostatize because of intellectual difficulties with the Christian faith. Rather the real enemy you have to fear is sin. Sin can lead us to fall away from the living God (Hebrews 3.12). So we have to guard against sin and daily rely upon the Holy Spirit to fill and empower us. I’m sorry to disappoint you, Bob, but there is no “100%-I-will-never-lose-it kind of faith.” Peter thought he had that kind of faith: “Though they all fall away because of you, I will never fall away!” (Matthew 26.33). Look where it got him. By contrast Paul didn’t have that kind of faith. He wanted to know Christ

that if possible I may attain the resurrection from the dead. Not that I have already obtained this or am already perfect; but I press on to make it my own, because Christ Jesus has made me his own. Brethren, I do not consider that I have made it my own; but one thing I do, forgetting what lies behind and straining forward to what lies ahead, I press on toward the goal for the prize of the upward call of God in Christ Jesus. Let those of us who are mature be thus minded (Philippians 3.11-15).

We should not, like Peter, have a presumptuous faith that we shall never fall, but rather be like Paul, trusting Christ daily for the power to persevere.

Fortunately, Scripture gives us advice on how to persevere. II Peter 1.5-11 exhorts us,

For this very reason make every effort to supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love. For if these things are yours and abound, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. For whoever lacks these things is blind and shortsighted and has forgotten that he was cleansed from his old sins. Therefore, brethren, be the more zealous to confirm your call and election, for IF YOU DO THIS YOU WILL NEVER FALL; so there will be richly provided for you an entrance into the eternal kingdom of our Lord and Savior Jesus Christ.

Cultivating these godly virtues in your life will be the best insurance of your perseverance to the end.