What is the ruling on someone who says to an atheist, after attempting to convince him: "Do not believe," and is this statement considered disbelief (kufr)? And what is the ruling on someone who says to an atheist who mocks Islam: "We are happy with backwardness"?

The one who told the atheist, "Do not believe," did not intend to approve of his disbelief, but rather to threaten and intimidate him, similar to the Almighty's saying: ﴿So whoever wills – let him believe; and whoever wills – let him disbelieve.﴾ Therefore, there is no blame or sin upon him for that. Likewise, saying, "We are happy with backwardness," is to cut short the arguments of the false ones. There is no blame in any of this, and beware of obsessive doubts in this matter.