The perseverance of the saints could more accurately be called the preservation of the saints, as this affirmation of the Westminster divines makes clear. The believer does not persevere through the power of his unaided will. God’s preserving grace makes our perseverance both possible and actual. Even the regenerated person with a liberated will is still vulnerable to sin and temptation, and the residual power of sin is so strong that without the aid of grace the believer would, an all probability fall away. But God’s decree is immutable. His sovereign purpose to save his elect from the foundation of the world is not frustrated by our weakness.
Sproul, R.C.; Grace Unknown: The Heart of Reformed Theology; Baker Books; Grand Rapids, MI; copyright 1997; p. 210