Paul uses irony here to show that those teachers are less important then people say there are. AT: "those teachers whom some think are better than anyone else" (See: [[rc://en/ta/man/translate/figs-irony]])
This negative phrase emphasizes the positive truth that he is trained in knowledge. The abstract noun "knowledge" can be translated with a verbal phrase. AT: "I am certainly trained in knowledge" or "I am trained to know what they know" (See: [[rc://en/ta/man/translate/figs-litotes]] and [[rc://en/ta/man/translate/figs-abstractnouns]])