John comes to the conclusion of his short letter in a resounding crescendo of assurance. He does this in the only possible context of biblical assurance, the context of the absolute difference between right and wrong.
"This is He who came by water and blood—Jesus Christ; not only by water, but by water and blood. And it is the Spirit who bears witness, because the Spirit is truth . . ." (1 John 5:6-21).
Many different interpretations have been offered for the meaning of "water and blood." The interpretation followed here is that John is referring to the baptism of Jesus, and the death of Jesus. Remember that the heresy being resisted is that of Cerinthus, in which "the Christ" was thought to have come on Jesus at His baptism and to have departed at the cross. Contrary to this, John insists the Lord Jesus Christ came through water and through blood. The Spirit bears witness to this. This is confirmed in the death of Jesus, when He was speared by a Roman soldier, and water and blood came from His side. Why did John record that particular detail? So that you might believe (Jn. 19:34-35).
What about the Johannine comma? Many Bibles do not have verse 7 and the first part of verse 8 in them. The controversy surrounding this involves enormous issues, which can only be touched on briefly here. First, there are objective grounds for controversy -- the comma is not in any Greek manuscript earlier than around the 16th century. Erasmus was forced to include it in his printed Greek text, and thus it found its way into the textus receptus. Unlike most supposed variant readings, good reasons exist for at least questioning this passage. At the same time, the issue is not not open and shut. The assumption is that an orthodox scribe inserted this phrase because the three witnesses reminded him of the Trinity. But if this is the case, why did he choose the unusual formulation of Father, Word and Spirit? The first citation of the verse was around 250 AD by Cyprian. It is cited by others and is in some Old Latin manuscripts of the 5th or 6th century. And the comma avoids a grammatical problem which occurs when the comma is left out. The controversy ought to be resolved within the Church because the Church is the pillar and ground of the truth, not the Bible societies, universities, publishing houses, Bible-mongers, etc. My operating assumption is that we should receive this passage as part of the canon of Scripture unless and until the Church gives her united testimony that it is spurious. Consequently we may say that we have two sets of witnesses. In heaven, we have the triune witness, and on earth, we find the testimony of the Spirit, and the baptism and death of Christ.
When God testifies to something, we have a moral obligation to believe it. We receive the testimony of men. Why not that of God (v. 9)? To refuse to do so is calling God a liar. The one who listens to the objective testimony has a subjective testimony within. The one who rejects the former will never get the confirmation of the latter (v. 10) because he has already called God a liar. So here is the point of divine testimony. The content of the testimony is that life is found in Christ (v. 11), and placed within us. But this is impossible apart from the Son (v. 12). And the point of this testimony is so that we would know that we have eternal life and would believe (v. 13).
Assurance of salvation is nowhere better displayed than in assurance in prayer. When we ask according to His will, we are heard. And when we are heard, we have what we have asked for. But the phrase according to His will should be used to indicate our submission, and not used as our escape clause to explain why our prayers are never answered (vv. 14-15).
What is the sin unto death? Remember the context of the letter. The sin unto death is that committed by those separating themselves from the truth. There is sin which leads to death, that which is away from Christ. While all unrighteousness is sin, some sin leads a man the way it led the antichrists who departed(vv. 16-17).
And last, the one born of God does not sin in this way (v. 18). He who walks in the right way is out of the devil's reach (v. 18). There are only two options: of God (v. 19), or under the evil one (v. 19). The Son of God has come and given us an understanding; moreover the Son of God is God (v. 20). And this is why we should keep ourselves from idols (v. 21).