1Jn 2:9-11 Anyone who claims to be in the light but hates his brother is still in the darkness. Whoever loves his brother lives in the light, and there is nothing in him to make him stumble. But whoever hates his brother is in the darkness and walks around in the darkness; he does not know where he is going, because the darkness has blinded him.
One of the most fundamental evidences distinguishing children of God from children of the devil which John speaks of throughout his letter is the evidence as to how one treats fellow Christians. There are false brethren in the Christian community who distinguish themselves by their unjustified hatred for legitimate Christians. Though they claim to be enlightened, and though they even claim to love fellow Christians, such people are lying if their lifestyle is inconsistent with their claims.
The reasons why they hate legitimate Christians is much the same as to why the world hated Christ. For example Jesus said to his unbelieving brothers, "The world cannot hate you, but it hates me because I testify that what it does is evil." John 7:7 The most fundamental reason why false brethren hate legitimate Christians is because of sin. "This is the verdict: Light has come into the world, but men loved darkness instead of light because their deeds were evil. Everyone who does evil hates the light, and will not come into the light for fear that his deeds will be exposed." John 3:19,20
Therefore, "Have nothing to do with the fruitless deeds of darkness, but rather expose them. ... But everything exposed by the light becomes visible" Eph 5:11,13 And by doing so you might just be able to distinguish genuine brethren from false brethren in your midst.