There is a big difference between thinking we know and knowing. For example, we may think our kids are not using drugs. However, many parents are horrified to learn their son or daughter is not only using drugs but has a serious addiction and has been supporting their habit by stealing.
Another example of thinking we know vs. knowing is when you're involved in a car accident. After the accident, you find out that the guy that hit you didn't have car insurance and you don't have uninsured motorist insurance. You thought you did.