What does the Bible say about God's work in believers?

The Apostle Paul expresses in Philippians 1:6 his confidence that the good work God begins in His people will be performed until the day of Jesus Christ. This reflects the doctrine of preservation, where God not only initiates the work of grace in a believer's heart but also sustains and completes it throughout their life. It emphasizes God's faithfulness and sovereignty, ensuring that those chosen by Him are secure in their salvation, as He actively works within them to shape their lives according to His purpose.