1 John 5:18
18 We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.
The Grace New Testament Commentary - 1 John 5:18
5:18 John now wishes to remind his readers that whoever is born of God does not sin, that is, the regenerate person as such is incapable of any sin. This leads to the further observation that he who has been born of God keeps himself with the result that the wicked one does not touch him. The inner man, born of God, has the inborn capacity to resist the pollution of evil and thus lies outside of Satan's reach.
In saying that the regenerate inward person (cf. Rom 7:22) keeps himself, John is not saying that one's inner self can somehow prevent all sin in the Christian life (cf. 1 John 1:5-10). What John means is that God's "seed remains in" the regenerate inner self (cf. 1 John 3:9) as the controlling element of his born-again nature and is impervious to even the slightest contamination from the wicked one. Believers' failures are due to the sinful "programming" of their earthly bodies, as Paul himself taught in Rom 7:7-25.
But try as he might, Satan cannot really touch the believer. But if a believer lets him, Satan will use his failures to lead him to further failures. So after every sin, a believer ought to rise from his confession to God, knowing that he is the same inwardly holy person he was before he failed!