What does 1 John 5:19 mean?

Both John and his readers "know that we are from God." Though it is true believers are created in God's image (Genesis 1:27), the context indicates the idea of salvation or being born again. Believers are children of God (John 1:12) and are from God spiritually and relationally.

John again notes that Satan has spiritual control over the entire world. John has already noted the believers he is writing to have overcome the evil one (1 John 2:13–14), mentioning Satan as the "evil one" also in 1 John 3:12 and 1 John 5:18. This letter has also spoken previously of the power of the world (1 John 2:15–17). Jesus prayed to the Father to keep believers from Satan (John 17:15). Believers are to use the armor of God against the evil one (Ephesians 6:16), knowing God will guard us against him (2 Thessalonians 3:3). The "world" is also mentioned in the first chapter of the Gospel of John, as something created by Jesus, and from which He takes away sins (John 1:9–10, 29).