"Take the helmet of salvation" (Eph. 6:17).
The key to conquering doubt is to focus on the preserving power of God.
Doubt comes to Christians in many ways. After you've sinned, your conscience might hiss at you, saying, "Surely you're not a Christian. Why would God save you anyway? You don't deserve His mercy. You're not good enough. How presumptuous to think God could ever use you!" Such doubts are common among Christians who focus on their performance rather than God's power.
All too often we're quick to acknowledge God's power to save us but slow to understand His power to keep us. To complicate matters, many Christians believe they can lose their salvation, so they live in constant fear of falling away from the faith. Still others have never learned what Scripture teaches about their security in Christ. They're so intent on pleasing God through their own efforts that they lose sight of grace and drift into a subtle works- righteousness mentality.
Your performance doesn't determine your standing in Christ; your standing in Christ determines your performance. Good works are the necessary result of salvation (Eph. 2:10) but they don't save you or keep you saved. That's God's work.
Jude said, "Now to Him who is able to keep you from stumbling, and to make you stand in the presence of His glory blameless with great joy" (v. 24). "Able" in that verse translates a Greek word that speaks of power. "Keep" literally means "to secure in the midst of an attack." "Stumbling" refers to falling into sin. Together they say that God is powerful enough to prevent you from stumbling into sin and falling away from Him—no matter how intense Satan's attacks might be. He will continue to protect and cleanse you until the day you enter His glorious heaven perfected.
Sin is a serious issue and you should never take it lightly. But when you do sin, remember that as a believer you're immediately cleansed by the blood of Jesus Christ (1 John 1:7). So always confess your sins and turn from them, but never doubt God's power or willingness to keep you saved. Trust in His grace, not in your ability to perform.
Suggestions for Prayer
Praise the Lord for continually cleansing your sin.
For Further Study
Memorize Jude 24-25 and recite it often as a reminder of God's power and majesty.