1. IX. Conclusion / Our Confidence (5:18-5:21)
Thematic Relation: From all that was discussed, we are confident of being His, and of being saved from the death-grip of sin.

Some Key Words

Know (eido[1492]):
the fullness of knowing, intuitive knowledge. This is the know of these verses except for the knowing of Him which comes from understanding (in v20).
Know (ginosko[1097]):
to know experientially; intimate knowledge; to be sure, understand. From Vine's: Ginosko tends to indicate a coming to knowledge, a beginning to understand, whereas eido tends to indicate a fullness of knowledge and understanding. This is the knowledge we are coming to have of Him who gives us understanding.
Given (didomi[1325]):
to grant, supply, furnish a necessary thing, deliver, to give one to someone as his own - to care for his interests
Understanding (dianoia[1271]):
intellectual faculty, mind, thought; a way of thinking and feeling
True ([228]):
genuine, real; having not only the name, but the real nature, and in that respect perfect
Guard (phulasso[5442]):
preserve from danger or harm, observe a commandment or law, beware; to protect, to avoid, shun free from
Idols (eidolon[1497]):
a form or appearance, an image or representation whether corporeal or mental, something set up to be worshipped as a god; whatever represents the form - either real or imaginary, a false god
 

Paraphrase

18 God keeps and protects His own. 19 We are confident in being of His own in enemy territory. 20 The Son teaches us of the Father, that we may abide in truth in God. 21We must let nothing usurp God's place in our lives.

Key Verse

5:20 - this is our assurance! That He has taught us, and keeps us in the truth.

Thematic Relevance:

 In His love for us, He protects us from the world's influence, and He has provided us with sound teaching in the words of His Son. This gives us a sure guide in our walk.

Doctrinal Relevance:

 19 The world is in rebellion against God. 20 Jesus is the Son of God. To know God and Jesus is to have eternal life.

Moral Relevance:

 We should be walking in full assurance of our future. We've been given the tools, the tests, and the promises necessary to be so assured.

Some Parallel Verses

5:18
Jas 1:27 - we should keep ourselves from being tainted by the world. Jude 21 - and in the love of God. Jn 14:30 - for the ruler of the world has nothing to do with God.
5:19
Jn 12:31 - but that ruler will be cast out in judgement. Jn 17:15 - And we must stay active in the world - protected from it by God - until the end. Gal 1:4 - In which ending, He will take us out of the world according to His will.
5:20
Jn 8:42 - Jesus proceeded from the Father.
5:21

New Thoughts

The final verse, which has always seemed a bit out of place to me, begins to fit. Perhaps what John is getting at is to point out that we should avoid the form or appearance of belief without the reality; that we need to protect ourselves from false conceptions of God and righteousness - and to hold fast to the clear truth of Christ.