The fourth angel poured out his bowl on the sun, and the sun was allowed to scorch people with fire.9 They were seared by the intense heat and they cursed the name of God, who had control over these plagues, but they refused to repent and glorify him. (Revelation 16:8-9 NIV)
Near the end of the tribulation period it will be so obvious that God is in control of events, that even those who rebelled against Him will have to admit the fact that He exists. When God strikes the sun, so that it’s heat is increased, the beast-worshippers will realize that God had done it. Nevertheless, at this point the are so strongly attached to their sin and rebellious ways that they refuse to “repent and glorify” God, even though the fact of God’s existence and sovereignty can no longer be denied. They would rather hate God than experience His love.
There is a difference between knowledge and faith. Having the knowledge that God created the world is not the same as having Faith in Him. These end-times rebels are still “unbelievers” even though they know for sure that the God of the Bible is the One true God. Yes, they know He exists, but they have not accepted Him. True, complete belief is more than a belief that God exists, but a complete submission of the heart to Him.