Anyone who is truly saved will be kept by the Holy Spirit, who is daily working to sanctify believers. Those who choose to walk away were never truly saved.
The Bible provides ample evidence that a genuine believer will never "give back" salvation. Romans 3:10-11 suggests that we don’t seek God but that He calls us. Only by God's grace does a person come to faith in Jesus (Ephesians 2:8-9). Only Jesus saves, not humankind’s works.
Also, those who truly have God's Spirit living within them will not turn against Christ. While every Christian continues to struggle with sin (Roman 7:24-25), those who have been changed by God's grace cannot be separated from His love(Romans 8:38-39).
While only God truly knows the heart of a person, the Bible indicates that there are two reasons someone might abandon the faith or at least appear to: Believers who are living in sin and struggling with doubts about salvation can appear to have lost or abandoned it. Second, someone who claims to "leave the faith" never had genuine faith at all (1 John 2:19). Jesus pointed out that some who say they know Him are actually unbelievers. It should not surprise us, then, when some today who claim to be Christians and later leave the faith were never truly Christians at all.
Return policies are usually pretty generous. You get thirty days, sometimes ninety, to give back what you bought. Salvation doesn’t work like that, though. The main reason is that we didn’t buy it, so we can’t return it.
Our salvation was bought with the blood of Christ. We didn’t pay for it, yet we get to keep it free of charge. That means we don’t have to live our lives worried about not doing enough good works or feeling insecure about eternity because we had a moment of unkindness. Even better, our salvation comes with a promise to remake us (2 Corinthians 5:17), so our behavior changes because we love Christ, not as payment for what has already been paid.
A Christian cannot "give back" salvation because this gift rests upon God's power, not our own. Instead, those who leave the faith are either those struggling who need our help or are those in need of genuine faith and the true gospel of Christ.