Both knowledge and confidence are important to achieve success. When we have the knowledge, we feel confident. More knowledge often increases confidence. Knowledge alone may not give us success.
Confidence comes with understanding, experience, and study. In other words, when you really get down to it, confidence comes with knowledge.
For example, I wouldn’t feel confident playing in a tennis tournament because I don’t know how to play well.
More simply put, a lack of knowledge tends to lead a person to greater confidence than is warranted. The over-confidence that a person projects lead people to believe that he/she is actually more knowledgeable than the person really is.