I John 5:20
In a Nutshell
In summary: it is all down to the appearance of Jesus Christ in whom we dwell.
* * * * *
Note how truth, love and life are all found in Jesus Christ.
20 And so we know that the Son of God has made his appearance and has thereby given us understanding that provides us with knowledge of the One who is true; and we continue to dwell in this One who is true, his Son, Jesus Christ. This One is none other than the true God who is eternal life.
John concludes His letter with a litany of statements that "we know". It began with him explaining why he had written (5:13). His aim was to confirm his readers in their knowledge of eternal life. With that confidence (5:14) they will know that their Father in Heaven wants to hear their requests and they will therefore know that He answers them completely whatever they may ask (5:15). Then it was a matter of praying for those who are in some kind of compromising position - recalling John's own problematic silence during Jesus' trial - in the knowledge that the One who is born of God does not offend (5:18). Then, writes John, we know that the entire world lies in the grip of the evil one (5:19), but from what he had written earlier we know that this world is passing away (2:17). God's children direct their love not to the world but to doing the will of our Heavenly Father. The One who makes that possible abides forever (2:17).
And that brings John to his conclusion: the knowledge completely depends upon the Son of God having made His appearance and in so doing He has bequeathed knowledge of the truth to those who trust Him and continue to dwell safe and secure in the shadow of the protective wings of the Lord (Psalm 90).
In Him and in Him alone can it be truly said that eternal life has been given.