Question: I think that what a philosopher would say about the eternality of the world is this: The heavens and the earth indeed had a beginning, as the Quran states. However, they were created by Allah from some prior substance that existed before them. So he would affirm a beginning to the heavens and the earth, but he would deny that before they were created nothing else existed besides Allah. Now my question is this: Why exactly is this belief kufr? Is it because of the hadith that “Allah was and nothing was with Him”?
Answer: It is a combination of many evidences, but it is basically because he believes that this substance is not created by Allah, and is His partner in eternity. The bottom line is that it is shirk.
[Shaykh] Abu Adam