I John 2

I John 2:1:
My little children, these things write I unto you, that you sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous:

“[I]t is evident that saints neither can nor dare adventure to plead their cause. Alas! the Judge is the almighty and eternal God; the law broken is the holy and perfect rule of God, in itself a consuming fire. The sin is so odious, and a thing so abominable, that it is enough to make all the angels blush to hear it but so much as once mentioned in so holy a place as that is where this great God doth sit to judge. This sin now hangs about the neck of him that hath committed it; yea, it covereth him as doth a mantle. The adversary is bold, cunning, and audacious, and can word a thousand of us into an utter silence in less than half a quarter of an hour. What, then, should the sinner, if he could come there, do at this bar to plead? Nothing; nothing for his own advantage. But now comes in his mercy-he has an Advocate to plead his cause-”If any man sin, we have an Advocate with the Father, Jesus Christ the righteous.”
— - John Bunyan (1628-1688), “The Work of Jesus Christ as an Advocate, Explained,” The Practical Works of John Bunyan, Vol. IV, p. 251

I John 2:2:
2 He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world.

The very fact that Christ suffered for us, and through His suffering became a propitiation for us, proves that we are (by nature) unrighteous, and that we for whom He became a propitiation, must obtain our righteousness solely from God, now that forgiveness for our sins has been secured by Christ’s atonement. By the fact that God forgives our sins (only) through Christ’s propitiation and so justifieth us by faith, He shows how necessary is His righteousness (for all). There is no one whose sins are not forgiven (in Christ).
— Martin Luther, Commentary on Romans (Grand Rapids, MI.: Kregel, 1976), 78

I John 2:19:
They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.

John gives us insight into what is going on in the lives of those who profess to follow Jesus but then turn away from him, depart from his Word and reject his people. When someone departs from the faith never to be recovered it is not that he has lost the salvation that was once possessed. Rather, such a person demonstrates by his departure that, no matter how loudly he professed to belong to the Lord’s redeemed, he did not genuinely possess the salvation that comes through faith in Christ. Though such people might be among us, they are not “of us.” Paul explains how this happens. How a person lives is determined by what he believes. Those who fall away do so because they become devoted to the “teachings of demons.” This does not mean that they get caught up in the occult. Rather, they come to believe notions that originate in hell and are consequently led away from the faith.
--Tom Ascol