You mentioned that the New Jerusalem will become the source of light for the earth. Wouldn’t the people on earth during the Millennium realize that it has replaced the sun and moon? How could there be any doubt about God among the nations if it is a visible light for them to see?
I don’t think there will be any doubt about God’s existence during the millennium. But neither is there any real doubt about God today. What passes for doubt is actually a refusal to acknowledge Him. In Romans 1:18-20 Paul said God has made His existence so obvious in the creation that mankind is without excuse. In the millennium it will be he same. The proof of God’s existence will be obvious, but people will still refuse to acknowledge Him.