Q. First John 5:18 seems to be saying that born again Christians don't sin. But can we ever really reach sinless perfection in this life?
A. The verse in question has sometimes been used to support a doctrine called entire sanctification and Wesleyan perfectionism. The verse says, "We know that whoever is born of God doesn't sin, but he who was born of God keeps himself, and the evil one doesn’t touch him."
In 1 John 5:18, John cannot be saying that we can be sinless in our actions. Why? Because he would be contradicting himself. In 1 John 1:8-10, he says, "If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and righteous to forgive us the sins, and to cleanse us from all unrighteousness. If we say that we haven't sinned, we make him a liar, and his word is not in us" (1 John 1:8-10). In other words, we recognize that we sin. If we say that we have attained to some sort of sinless perfection, we are liars. But we also know that as believers we are forgiven and cleansed from our sin.
Because we are in Jesus Christ, His sacrifice has paid the penalty for our sins and God has imputed His righteousness to us. Paul says,
There is therefore now no condemnation to those who are in Christ Jesus, who don't walk according to the flesh, but according to the Spirit. For the law of the Spirit of life in Christ Jesus made me free from the law of sin and of death. For what the law couldn't do, in that it was weak through the flesh, God did, sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh; that the ordinance of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.
This has nothing to do with some sort of sinless perfection that we attain, but it has everything to do with what Jesus has done for us (see also Isaiah 53:11; Jeremiah 23:6; Romans 5:18; and 2 Corinthians 5:21).
Returning to the words of John, notice what he says in 1 John 3:5-6: "You know that he was revealed to take away our sins, and in him is no sin. Whoever remains in him doesn't sin. Whoever sins hasn't seen him, neither knows him" (1 John 3:5-6). The Greek word translated "remains" is menō. Its translation here as "remains" can give a false impression that it is possible for someone to be in Christ and then leave. But menō also means "abide." John is saying that those who abide in Christ don't sin. This is not because of "works of righteousness, which we did ourselves" (Titus 3:5), but because of being in Christ. And John says in 1 John 3:6 that those who do sin don't know Christ. The difference between those who sin and those who don't is that those who don't sin know Christ.
So, in 1 John 1, John says that we Christians sin, and in 1 John 3, he says we don't sin. How can he say these two seemingly contradictory things? In chapter one he is talking about what we ought to know from our perspective. If we assess ourselves honestly, we know that we still sin. But in chapter 3, John is writing of God's perspective. God doesn't see our sin; He sees the sinlessness of Christ. Even though we sin, it is impossible for us to undo what Christ has done for us: "Whoever is born of God doesn't commit sin, because his seed remains in him; and he can't sin, because he is born of God" (1 John 3:9). First John 3:9 would be a direct contradiction of 1 John 1:8-10 unless we understand John's shift in perspective. Because of Christ, God does not count the Christian's sinful actions as sin. As we read earlier, "There is therefore now no condemnation to those who are in Christ Jesus...." (Romans 8:1). Thus, from God's perspective, the Christian doesn't sin. "Blessed is the man whom the Lord will by no means charge with sin" (Romans 4:8).
And so, John can say in 1 John 5:18, "We know that whoever is born of God doesn't sin, but he who was born of God keeps himself, and the evil one doesn't touch him." The evil one is Satan, the accuser of the brethren (Revelation 12:10). He can certainly try to accuse us, and sometimes he persuades us to believe it and become discouraged. But the devil cannot cause God to bring a guilty verdict against sinners who are born of God, and, therefore, dwelling in their Savior. This is not because they live a perfect life but because God declares them to be sinless in Jesus Christ.
But now the righteousness of God has been manifested apart from the law,
although the Law and the Prophets bear witness to it—the righteousness
of God through faith in Jesus Christ for all who believe. For there is
no distinction: for all have sinned and fall short of the glory of God,
and are justified by his grace as a gift, through the redemption that is
in Christ Jesus, whom God put forward as a propitiation by his blood, to
be received by faith. This was to show God's righteousness, because in
his divine forbearance he had passed over former sins. It was to show
his righteousness at the present time, so that he might be just and the
justifier of the one who has faith in Jesus.
Romans 3:21-26, English Standard Version