"If in preaching the gospel you substitute your knowledge of the way of salvation for confidence in the power of the gospel, you hinder people from getting to reality"

The quote means that if someone depends on their own understanding and knowledge about how to be saved, rather of having faith and rely on the power of the gospel of Jesus Christ, they will hinder others from truly finding and experiencing the fact and power of the gospel. Essentially, relying solely on one's own understanding and understanding restricts the impact and efficiency of the gospel message.