Paul wrote of those who deceive the hearts of the simple (Romans 16:18), and then wrote,
“For your obedience has become known to all. Therefore I am glad on your behalf; but I want you to be wise in what is good, and simple concerning evil” (Rom 16:19).
The Christians at Rome had obeyed the gospel, and for that Paul rejoiced, but he wanted them to grow in wisdom concerning what is good, and to grow simple concerning what is evil. We should be wise when it comes to the teaching of Christ, but we should be simple when it comes to experience in evil, that is, the longer we live, the less we should commit evil.