Commentators explain that when Paul admits to being “rude in speech,” he isn't saying he's uneducated. Rather, he's conceding that he doesn't use the polished, professional rhetoric prized by Greek culture. He intentionally chose a plain, common style to ensure the gospel message itself—the “knowledge”—was clear and powerful, rather than being obscured by fancy words. His focus was on the truth of the message, not the elegance of its delivery.