Because God gives us eternal life through Jesus Christ, the true believer is secure in that salvation for eternity. If one has been genuinely saved, they cannot lose it. Salvation is maintained by the grace and power of God, not by the self-effort of the Christian. It is the grace and keeping power of God that gives us this security.
John 10:29; 2 Timothy 1:12; Hebrews 7:25, 10:10, 14; 1 Peter 1:3-5