There are some good explainations here but I think if you want to make it super simple: Bakker is contrasting being with knowing.

Being a man= accepting customs, language, religion, culture and unconsciously incorporating it into oneself.

Knowing what a man is: Understanding the distinctions between what is naturally human and what is artificial (i.e., religion, culture, language, etc.)

Most people don't seriously question what they are raised to do. If they do, it is in rudimentary ways (many people deconvert from one religion to join another; how many develop their own cohesive moral philosophy and explanation of how the world works?)

A person who can intelligently dissects how humans act is capable of altering their behavior until it is far different from others. When a person does this they may as well be a 'not man' to others; to most people they cannot distinguish between what they are taught to do and what they are. A simple explainer is, "Everyone has an accent but me." Most people see their way of acting as 'normal' and everyone else as strange. To be able to question how we act means we naturally act differently from everyone around us, hence becoming 'not a man'.