Fourth. The reason why the Apostle John was telling his addressees about the divinity and the humanity of the Lord Jesus, was so "that you too may have fellowship with us"5509 — and so "that your joy may be full."5510 Indeed, may the same be true also of all modern Christians!
Fifth. This fullness of joy could be obtained completely, by repenting of one's sins and believing on the Lord Jesus5511 as "the Lamb of God Who takes away the sin of the World."5512 For "if we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness" (which is the breaking of His Ten Commandments).5513
No sin of a true believer was or is too vile to have been washed away on Calvary and forgiven by the Father. For "the blood of Jesus Christ, His Son, cleanses true believers from all sin — both the sins of their past, as well as all their present and future sins.
In addition, however, because that once-shed blood of Jesus shall never lose its power — it even now keeps on cleansing true believers from all sin (alias the ongoing transgression of God's most holy Law).5514 For "if any man
sin" or if anybody commits a sin — we [Christians] have an Advocate with the Father, Jesus Christ the righteous [Lawkeeper]; and He is the Propitiation [or the complete atoning Sacrifice] for our sins."5515
Let it always be remembered what Jesus said when He died on the cross. As the only sin-bearer for the sins of His children, He then cried out: "It is finished!"5516
Last. The Apostle assured His addressees that Jesus died not only for the sins of the Apostles and for the sins of those who believed in Him through the word of the Apostles. For Jesus died for the sins of all who believe in Him. Indeed, He died even for the sins of all who not yet believe in Him but who will yet repent and believe in Him.
Accordingly, "He is the Propitiation for our sins — and not for ours only, but also for the sins of the whole World."5517 For Jesus the eternal Son of God was no mere Son of Judah — no mere tribal Saviour of the Jews alone. To the contrary. He was indeed the Second Adam alias the Son of man and the Saviour of the World5518 — the Saviour of all Nations everywhere.5519 Indeed, the very ends of the Earth would yet turn to Him.5520
Jesus died for a vast multitude.5521 Indeed, even if He had intended to redeem every single human being who ever lived and who ever shall live — He would not have suffered any more than He did. Thus, the shedding of His most precious blood5522 was certainly sufficient to save the entire human race.
But still more glorious. The shedding of His blood was and is not just sufficient. It is also efficient. Indeed, it effectively saves not just many of the Jewish Nation5523 but also many of the whole World5517 — namely that vast multitude from every Nation5524 for whom Christ died.
So, "if any man sin — we have an Advocate with the Father, Jesus Christ the righteous."5515 For, as He Himself has said, "all that the Father gives Me, shall come to Me; and him who comes to me, I shall no way cast out."5525
So Jesus Christ is indeed "the propitiation…for the sins of the whole World."5498 This means, as the Geneva Bible comments, that Christ is the reconciliation "of them who have embraced the Gospel by faith in all ages, degrees, and places. For there is no salvation without Christ."