This double negative emphasizes that what Christ has accomplished is the only thing that Paul wants to speak about. Alternate translation: "I will dare to speak only of what Christ has accomplished" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])
This can be translated in active form: Alternative translation: Alternate translation: "These are things that Christ has accomplished through what I have said and done" (See: [[rc://en/ta/man/translate/figs-activepassive]])