John said one born of God cannot live in sin whoever sins has never known or seen him
so again, You make people out to be liars.
The book of 1 John is laying out criteria for testing spirits (1 Jn 4:1). Specifically, the gnostic heretics taught "because the material world is inherently sinful" :
1) gnostic Christians never stop sinning, they will continue living sinfully
2) Christ didn't incarnate, because, if He had, He would've been sinful.
Thus John, addressing that heresy, says,
1) The one born of God does not continue in sin, and then stipulates "because His seed remains in him", and
2) Every spirit that confesses Christ has come in the flesh is of God.
However,
1. The heretics who taught the Galatians the false gospel that had to serve by the Law of Moses certainly taught that Christ had come in the flesh and had died for their sins, and yet Paul said they were teaching doctrine of demons ("This persuasion doesn't come from Him Who calls you"), so we know that not everything John says is to be taken as you are wanting it to be taken, "across the board", but that some of it is specifically aimed at specific heresies.
2. We know that not all remain in Him (1 Jn 2:28), and His Word in them (Jn 15), so, if they do not remain in Him, and His Word in them, God, as Ezekiel 3:20 says, "forgets" their righteousness (of faith), and their name is blotted out of God's Book ("the one who has sinned against Me I will blot out of My Book"), so that "I never knew you" and "they were never of us". It is as though they were never saved.
Those who remain in Him and His Word in them do not remain in sin, but not all remain in Him and His Word in them, as the parable of the sower shows (and these different heart conditions toward God's Seed/Word are not static conditions, the heart conditions can change--as Heb 3:12,13 says, "become hardened through the deceitfulness of sin"--they just describe different states of receptivity to God's Word that hearts can be in). This is the selfsame reason Paul says he himself can fall away from the faith if he doesn't make his body his slave: sin is deceitful, and when sin puts its deceit into the heart, the heart, ipso facto, doesn't harbor truth, and it ceases to be soft, loving, it becomes hardened, unloving and unbelieving ("love believes all things")--he himself could be deceived by sin and be "disqualified" for the "race" that is faith, and not be eligible for the life repaid those who live righteous lives through faith in Christ (Ro 2:6-16; Gal 6:6-10) not through the Law (Php 3:9).
We also know God has disinherited His children (Dt 14:1) before, and specifically for sinning (Dt 32:5; Hos 1).