The phrase “the perseverance of the saints” describes the biblical teaching that, once a person is justified by faith alone in Christ alone, they are forever justified. This is because Jesus Christ preserves us. He, in fact, perseveres with us—and so we persevere to the end (Jude 24–25).
Those who have been born again by the grace of the gospel of God are assured that they will be increasingly sanctified and ultimately glorified. We will persevere to the end and be finally and fully saved. But the reason, fundamentally, is because of the perseverance of the Saviour: Jesus will complete what He has begun. This conviction will go a long way toward helping us to persevere in the face of difficulties as we seek to be faithful and fruitful disciples of the Lord Jesus Christ. This is the theme of Psalm 129.