I am convinced that once a person sincerely and honestly trusts Christ for his or her salvation, they become a member of God's family forever—and nothing can change that relationship. Jesus said, "My Father, who has given them to me, is greater than all; no one can snatch them out of my Father's hand" (John 10:29).
Does this mean it doesn't matter how a professing Christian lives, since they will be saved anyway? No, not at all! For one thing, if that person refuses to follow Christ and lives a life of sin, it may well mean he or she wasn't a Christian after all. The Bible warns, "No one who lives in him keeps on sinning" (1 John 3:6). Outwardly Judas was a disciple—but inwardly his heart was rebellious and unbelieving.
In addition, sin separates us from God, and when we sin our fellowship with God is broken, and our witness for Christ is blunted. Peter stood at the fire and denied Jesus three times, and those around him must have assumed Jesus meant little to him (see Mark 14:66-72).
But listen: Peter repented, and God forgave him and restored him to usefulness. And that can happen to you. Don't let sin hold you in its grip any longer, but repent and ask Christ to forgive you and help you—and He will. His promise is for you: "If we confess our sins, he ... (will) purify us from all unrighteousness" (1 John 1:9).