Why do Christians still sin after salvation?

from the old testament

• Genesis 3:6 records how sin entered the world: “So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate.” Adam, as head of the first human household and recipient of God’s direct command, was held responsible for introducing sin into humanity (Romans 5:12).
• Leviticus 4 and 5 discuss God’s provisions in the Mosaic Law for unintentional and intentional sin. Leviticus 5:17–18 says, “If anyone sins, doing any of the things that by the LORD's commandments ought not to be done, though he did not know it, then realizes his guilt, he shall bear his iniquity. He shall bring to the priest a ram without blemish out of the flock, or its equivalent, for a guilt offering, and the priest shall make atonement for him for the mistake that he made unintentionally, and he shall be forgiven.” The Law was provisional until Christ became the ultimate sacrifice for all sin.

from the new testament

• Romans 7:21–23 shows that even Paul, who devoted his life to the gospel, still struggled with sin: “So I find it to be a law that when I want to do right, evil lies close at hand. For I delight in the law of God, in my inner being, but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members.” He goes on to write, “Thanks be to God through Jesus Christ our Lord!” (v. 25) for freeing him and all believers from eternal damnation because of our failure to maintain righteousness. All believers are redeemed through Christ.
• Philippians 1:6 suggests that God works in our lives to make us Christ-like: “And I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ.” That “good work” God began is the process of sanctifying us while on this earth. Until we’re resurrected, believers will struggle with sin, but we are helped by the indwelling Holy Spirit (Galatians 5:16).
• Ephesians 6:11 shows that sin is still a battle for believers but that we can conquer it through the means God has provided: “Put on the whole armor of God, that you may be able to stand against the schemes of the devil.” Though we are no longer “slaves of sin” (Romans 6:17), we are still Satan’s target (1 Peter 5:8) and must guard against that.

implications for today

“Be good”—mothers say that to kids when sending them off from home. Adults say it to other adults, usually with a sly wink that gives the advice a different meaning. In either case, those words aren’t likely to carry much weight. That’s because it’s too easy to “be bad.” Even a person with the nicest temperament can’t be that way all the time. But Christians become “new creations” when we accept Christ as savior (2 Corinthians 5:17). We’re covered in Christ’s righteousness before the Father. God works in our lives, through the Holy Spirit, to conform us to the image of His Son. We are butterflies but are still confined in our cocoons. Are you a Christian who wants to “be good” but still falls into sin? If so, are you praying about it, meditating on God’s Word daily, and getting support from your church? God has given us the means to defeat sin and has promised not to allow more temptation than we can bear (1 Corinthians 10:13). We have the tools to overcome the temptations of the world, the flesh, and the devil. We must use them. Christians may still fall into sin, but our lives should not be characterized by sin (Romans 6:1–2; Galatians 5:16–17; 1 John 3:6).