1John 5:18,19
vs 18 We have perceived that whoever has been born of God does not characteristically sin; but he who had been born of God at some point characteristically keeps himself, and the wicked one does not touch him for himself.
vs 19 We have perceived that we are of God, and the whole world lies itself under the sway of the wicked one.
John reiterates the fact that those born of God do not continue to sin, just as he said in 1John 3:9. (So if they do, as the "brother" in vs 16, such people have not been born of God.) Bringing this up here supports the idea that the previous two verses were referring to nominal Christians - those who continue to sin.
And as we also read in 1John 3:9 the reason why those born of God don't continue to sin has to do with the nature of the rebirth. "No one who is born of God will continue to sin, because God's seed remains in him; he cannot go on sinning, because he has been born of God." John alludes to this idea here in the phrase "the one who was born of God keeps him safe, and the evil one cannot harm him.", though there are nuances of interpretation.
For example, "the one who was born of God" could refer to Jesus since "was born" is in the aorist indicating an historic past event as opposed to the perfect tense which John uses earlier in the verse to referring to "anyone born of God". And Jesus said, "While I was with them, I protected them and kept them safe by that name you gave me." John 17:12 and "My prayer is not that you take them out of the world but that you protect them from the evil one." John 17:15, not to mention also the analogy he makes in John 10 of himself being a shepherd taking care of his sheep, and in particular John 10:28 "I give them eternal life, and they shall never perish; no one can snatch them out of my hand." (well I guess I mentioned it anyway!) And this is the most likely interpretation based upon the Greek manuscripts upon which many modern translations, like the NASB and NIV are based.But there is a textual variant in some manuscripts, which is carried over in the KJV where the word "him" in "keeps him" is actually "himself", which means this could be read that those born of God keep (take care of) themselves such that they don't continue to sin.
In fact it could be argued that both of these are the case, kind of like Paul said, "Therefore, my dear friends, as you have always obeyed— not only in my presence, but now much more in my absence-- continue to work out your salvation with fear and trembling, for it is God who works in you to will and to act according to his good purpose." Php 2:12,13
In any case, those born of God are safe from Satan, though the rest of the world is under his control, just as John has previously indicated, "everyone born of God overcomes the world." 1John 5:4a The word "harm" is more literally "touched" (to fasten one's self to, adhere to, cling to). The sense is that while Satan may in fact touch us from time to time, but he has nothing to hold on to. So he can't cling to us as he can to others in the world.