When Paul said that he "decided to know nothing" he exaggerated to emphasize that he decided to focus on and teach about nothing other than Jesus Christ. Alternate translation: "I decided to teach nothing ... except Jesus Christ" or "I decided not to teach anything ... except Jesus Christ" (See: [[rc://en/ta/man/jit/figs-hyperbole]])
The double negative here emphasizes that Jesus Christ was the only thing Paul wanted to think about. Alternate translation: "when I was among you to know only Jesus Christ" (See: [[rc://en/ta/man/jit/figs-doublenegatives]])