Certainty is a funny word.

To be sure of yourself is an enormous responsibility. You have to make sure you don't transgress the boundaries of certainty and conceit. You have to draw the line between confidence and arrogance.

To get even near that stage of self assurance, is not easy to begin with. It requires standing by principles regardless of any obstacle, it requires tremendous knowledge and exposure. It requires a lot of consideration.

I've met very few people who are very sure of themselves. Out of the few, only 1 or 2 are substantial.
The rest, are just misconceptions.