Assurance

Perseverance of the Saints...Our Joy and Confidence in Salvation

Once God has saved someone from their sin, he continues to keep and preserve them by his power and grace and will never let them go. Thus, they persevere to the end and can never be lost. If God did not do this, we would inevitably turn back again to the world, because of the sin that is around us and within us. Thus God enables his children to continue in faith and obedience throughout their earthly lives, then to pass into God's presence forever.

This doctrine is not to be taken as a license to go on sinning, as if the believer is free to act in any way he chooses now that he is eternally secure in Christ Jesus. The true believer will show signs of a growing desire for holiness and an increasing loathing of sin. The one who attempts to use the grace of God as an excuse for sinful living is in all probability not a true believer, for where there is spiritual life, the fruit of the Spirit will become evident.
----------------------------------------------------------------------------------

The perseverance of the saints means that all those who are truly born again will be kept by God’s power and will persevere as Christians until the end of their lives, and that only those who persevere until the end have been truly born again.
Wayne Grudem from Systematic Theology (pg. 788)

If our religion be of our own getting or making, it will perish; and the sooner it goes, the better; but if our religion is a matter of God's giving, we know that He shall never take back what He gives, and that, if He has commenced to work in us by His grace, He will never leave it unfinished.
C.H. Spurgeon

For non-reformed theologies..."at the end of the day, the security of the believer finally rests with the believer. For those in the opposite camp [Reformed], the security of the believer finally rests with God -- and that, I suggest, rightly taught, draws the believer back to God himself, to trust in God, to a renewed faith that is of a piece with trusting him in the first place."
D.A. Carson

Click here for many other resources on this doctrine.

Scripture (hover over or click the reference)
1 Samuel 2:9; Job 17:9; Psalm 31:23; Psalm 32:7; Psalm 34:7; Psalm 84:5-7; Psalm 89:30-33; Psalm 94:14; Psalm 97:10; Psalm 121:7; Psalm 125:1; Proverbs 2:8; Proverbs 4:18; Isaiah 54:4-10; Jeremiah 31:3; Jeremiah 32:38-42; Ezekiel 11:19-20; Matthew 7:24-27; Matthew 18:12-14; Matthew 24:22-24; Luke 6:46-49; Luke 22:31-32; John 5:24; John 6:37-40; John 6:44; John 6:51; John 8:31-32; John 10:3-5; John 10:14-16; John 10:26-30; John 15:16; John 17:1-2; John 17:9; John 17:11; John 17:15; John 17:24; Romans 6:1-4; Romans 7:24-8:4; Romans 8:28-39; Romans 11:28-29; Romans 14:4; 1 Corinthians 1:4-9; 1 Corinthians 3:13-15; 1 Corinthians 10:13; 2 Corinthians 1:21-22; 2 Corinthians 4:8-9; 2 Corinthians 4:14; 2 Corinthians 5:5; 2 Corinthians 5:17; Galatians 2:19-21; Galatians 6:15; Ephesians 1:11-14; Ephesians 2:8-10; Ephesians 4:30; Philippians 1:6; Philippians 1:19; Philippians 2:12-13; Colossians 3:1-4; 1 Thessalonians 5:23; 2 Thessalonians 3:3-5; 2 Timothy 1:12; 2 Timothy 2:19-21; 2 Timothy 4:18; Hebrews 2:18; Hebrews 7:25; Hebrews 10:14; Hebrews 10:36-39; Hebrews 12:2; Hebrews 13:5; 1 Peter 1:3-5; 1 Peter 2:25; 1 Peter 5:10; 2 Peter 1:10; 1 John 2:13-14; 1 John 2:19-20; 1 John 3:9; 1 John 4:4; 1 John 5:4; 1 John 5:11-13; 1 John 5:18; Jude 1; Jude 24-25