How can John say that love is a mark of believers and hate is a mark of non-believers when we know believers who struggle with hate and non-believers who genuinely love others?
No true believer is committed to sin as a lifestyle because God plants in believers a longing for holiness and righteousness which opposes any tendency to rebel. Believers no longer remain committed to a lifestyle of sin.
1 John 2:18-28 tackles the question: how do you know what’s essential and what is non-essential?
John sets the world and the gospel in opposition to each other. You can’t serve them both. You have to choose and his first exhortation is don’t choose the world.
How should we handle theological disagreements among believers? What issues are so critically important that we should draw a line? And, on what issues should we agree to disagree? John gives us a place to draw the line.