Sometimes we have new people come through our church for weddings or meetings and they'll leave their purses and valuables unattended. In fact I think that the church has been a bit of a breeding ground for naivete over the years. Naivite likes to creep into the beliefs and notions and faith-lives of people and take up residence. And Jesus was conscious of this and he warned against naivete in the faith. It's a warning to keep an eye out.
Evil is very real and dangerous.
Don't be the victim of your own naivete. Now in today's passage the issue at hand was false prophets - people masquerading as holy, pretending to have a message from God but in fact having their own interests at heart. And he says the way we can tell whether they are legitimate or imposters is by their fruit. A good tree will bear good fruit. A bad tree cannot give good fruit.