Church people can claim a lot of things. They can claim to ‘know him’ (v4), to ‘live in him’ (v6) and now in v9 ‘to be in the light’ but for it all to be false. The reason being is somethings are just impossible.
“Anyone who claims to be in the light but hates a brother or sister is still in the darkness. Anyone who loves their brother and sister lives in the light, and there is nothing in them to make them stumble. But anyone who hates a brother or sister is in the darkness and walks around in the darkness. They do not know where they are going, because the darkness has blinded them.” (1 John 2 v 9-11)
Written to those within the Church:-
It is impossible to be in the light and hate at the same time.
It is impossible to get out of the darkness if you hate.
It is impossible to love and not be in the light.
It is impossible to stumble when there is nothing to sin about.
It is impossible to hate and not be in darkness.
It is impossible to receive guidance when you are blinded by darkness.
The problem is not only whether you love but whether you hate.
For John the problem was within not outside the Church.