What is meant by His, the Almighty's, saying: "Have not those who disbelieved seen that the heavens and the earth were a joined entity, and We separated them, and We made from water every living thing? Then will they not believe?", and does it mean seeing the cosmic sign after creation, especially since there was no creature to see when the universe was created?

The "seeing" (ru'yah) in the verse, "Have not those who disbelieved seen that the heavens and the earth were a joined entity, and We separated them and made from water every living thing? Then will they not believe?" (Quran 21:30), is a seeing of the heart, meaning knowledge and insight. The meaning is: Have not the disbelievers known that the heavens and the earth were connected, and Allah separated the heaven with rain and the earth with vegetation, and made from water every living thing? This is indeed proof of Allah's power and oneness.