 So, the Father teaches Catherine there's no sin that doesn't harm others. What the Father calls secret sin, even those things that seem like they're only interior to the person's heart, will always lead us to fail to love others as we should and will eventually end up in open sin that directly attacks the good of another. So, for example, if a person is losing their faith in God, it will eventually come to a manifestation in harming others, or that if a person does something in private that they think is just about themselves, it will cause a harm to the others around them.