How can the grammatical authenticity of the Qur’an be judged if many of the grammatical rules were derived from it?

For the believer, it is sufficient that the Quran is from Allah for him to be certain of the authenticity of every letter in it. As for the non-believer, it is sufficient to argue against him that the Arabs, despite their eloquence and their motivation to oppose, did not challenge the language of the Quran. Rather, they acknowledged its elevated status, as in the story of Walid ibn al-Mughirah, who described the Quran as sweet and enchanting, saying that it rises above all and nothing can rise above it. However, he was forced to describe it as magic under pressure from his people.