Well kiddo, you know how we all make mistakes sometimes? Like when we forget to clean up our toys or we say something mean to our friends? Well, imagine that there was a really, really big mistake that we could make that would be really, really bad. This mistake is called an eternal sin.
An eternal sin means doing something so bad that it makes God very sad and angry, and it can never be forgiven. It's like a stain that can never be washed out. The Bible talks about what kinds of things can be considered eternal sins, like denying God or hurting someone in a really, really bad way.
Now, it's important to remember that we're all human and we all make mistakes sometimes. But we can always ask God to forgive us when we do make mistakes, and he will. But if we purposely and continually do something that we know is really, really bad, that is what's called an eternal sin, and we don't want to do that. We want to be good and make God happy.