What is meant by knowledge in al-Shafi'i's statement: "Debate the innovators with knowledge; if they acknowledge it, they are defeated, and if they deny it, they are declared disbelievers"

Question:
Fear Allah, Your Eminence the Shaykh. The questioner says: In al-Shafi'i's statement (may Allah have mercy on him), "Debate the innovators with knowledge; if they acknowledge it, they are defeated, and if they deny it, they are declared disbelievers," what is the knowledge that, if denied, leads to the denial of its denier?

Answer:
The knowledge of Allah (Exalted and Majestic is He). If a person says that Allah is not All-Knowing, he is a disbeliever because he is lying about Allah and His Messenger and the consensus of the Muslims. Al-Shafi'i's statement is regarding the Qadariyyah, the Qadariyyah who deny divine decree. So if they say that Allah did not know things before they occurred, they are disbelievers for this, because they denied one of His attributes (Glorified and Exalted is He).