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.
These are difficult verses. Sometimes it is so much easier to love those outside the kingdom than those within it. I make less allowances for Christians. I am more judgemental of Christians, more easily offended by Christians. I am not sure why this is.
But it is true.
And John says that harboring these kind of grievances, clutching at grudges, hoarding small offenses will only keep us in the dark. We will stumble: tripping over ourselves, tangled up in negative thought processes.
The word for stumble here is "skándalon", the trigger of a trap. Our hatred and bitterness, our cynicism and judgemental attitudes will cause us to fall. We are ensnared by our own wrong thinking, blinded by our biases, caught up by complaining.
And what is worse, to do so is to betray our identity. It is to belittle the cross and blur that which should make us distinct. It is by the way that we love that we are recognized as belonging to Jesus. (John 13.35) Failure to love our brothers and sisters properly doesn't just damage our own reputation; it damages his.