How terrible is sin?
It didn't just cost Achan his life, but the lives of his family members. And it wasn't just the family of Achan, but 36 other warriors who had gone to fight against Ai.
If there is a fight worth fighting, it's against sin. But it's not just for my sake (though that would be enough). It's for the sake of those around me.
As a husband, my sin can affect my wife. Even the sin which I have hidden, "with the silver underneath," will weasel its way out. Even the part which I think I've got buried so deep no one will ever know about - thoughts, intentions, feelings, admirations, desires, tinges of allegiance - even those find their way to the surface, like a flower growing from the smallest crack in the sidewalk.
As a dad, my sin can affect my kids. Attitudes work their way out in sharp words or sharp actions. Instead of passing along the fruit of the Spirit, I end up spiritually genetically engineering a person for whom sin is no big deal. If I can dismiss it, so can they.
As a pastor, I can lack spiritual power because of sin.
As a friend, I can run out of compassion or service that is needed.
You fill in the blank here. But it's serious. And it's a war. And it's worth fighting.