5:18 sinneth not. See notes on I John 3:6-10.
5:18 keepeth himself. “Keepeth” is used in the sense of “guardeth.” The born-again Christian will (or at least should) guard himself against the deceptions of Satan by constantly availing himself of “the whole armour of God” (Ephesians 6:10-18), by consciously resisting the devil steadfast in the faith (James 4:7; I Peter 5:8-9), and by following Christ’s example in silencing him with appropriate Scriptures (Matthew 4:1-11). To the extent we thus guard ourselves against the wiles of the devil, that “wicked one” cannot touch us, for “greater is He that is in [us], than he that is in the world” (I John 4:4).