The short answer
Salvation is a gift to be received, lived, and faithfully completed. The New Testament joins assurance in God's grace with urgent calls to perseverance.
Jesus said
Christ warns His disciples to remain watchful, bear fruit, and abide in His love.
John 15:1–10Matthew 24:13Revelation 2:10
The apostles taught
Paul describes the Christian life as a race and urges believers not to receive grace in vain.
1 Corinthians 9:24–27Philippians 2:12–132 Timothy 4:7–8
The Catholic synthesis
Final perseverance is a grace. Christians therefore hope in God, pray, receive the sacraments, repent when they fall, and continue following Christ.