Sin can be like germs. Just because you can’t see it doesn’t mean it isn’t there. It only takes a little bit to make a surface unusable or a food inedible. It only takes a little to make you and those around you sick. If you don’t clean properly with something that kills it, it will only continue to grow and infect.
On top of that, human immune systems are notoriously bad with sin. Sin is like having an aggressive infection; it doesn’t go away on it’s own, you need a consistent dose of antibiotics, and you can only get those from the doctor. We need to see the Great Physician, Jesus Christ, in order to deal with sin properly.
Related:
Why Is Sin Such a Big Deal?
Still a Big Deal
A Bit on Sin
Snaking Sin