A.T. Robertson Commentary 1 John 5:18

"We know that whosoever is begotten of God sinneth not; but he that was begotten of God keepeth himself, and the evil one toucheth him not." — 1 John 5:18 (ASV)

We know (οιδαμεν). As in 3:2,14; 5:15,19,20. He has "ye know" in 2:20; 3:5,15.

Sinneth not (ουχ αμαρτανε). Lineal present active indicative, "does not keep on sinning," as he has already shown in 3:4-10.

He that was begotten of God (ο γεννηθεις εκ του θεου). First aorist passive articular participle referring to Christ, if the reading of A B is correct (τηρε αυτον, not τηρε εαυτον). It is Christ who keeps the one begotten of God (γεγεννημενος εκ του θεου as in 3:9 and so different from ο γεννηθεις here). It is a difficult phrase, but this is probably the idea. Jesus (John 18:37) uses γεγεννημα of himself and uses also τηρεω of keeping the disciples (John 17:12,15; Revelation 3:10).

The evil one (ο πονηρος). Masculine and personal as in 2:13, not neuter, and probably Satan as in Mt 6:13, not just any evil man.

Touchest him not (ουχ απτετα αυτου). Present middle indicative of απτω, elsewhere in John only Joh 20:17. It means to lay hold of or to grasp rather than a mere superficial touch (θιγγανω, both in Col 2:21). Here the idea is to touch to harm. The devil cannot snatch such a man from Christ (John 6:38f.).