As an atheist, one possible response to the necessary being argument or the question of what caused the Big Bang could be as follows:
Regarding the necessary being argument: Atheism generally rejects the existence of a necessary being as posited in many philosophical arguments for the existence of God. Atheists typically approach the question from a standpoint of empirical evidence and reason. They may argue that the concept of a necessary being is speculative and lacks sufficient evidence to support its existence. They may also contend that the burden of proof lies with those making the claim, and until compelling evidence is provided, they see no reason to accept the notion of a necessary being.
Regarding the cause of the Big Bang: Atheists often acknowledge that the origins of the universe and the cause of the Big Bang are currently areas of ongoing scientific inquiry and investigation. While the Big Bang theory describes the expansion of the universe from a highly dense and hot state, it does not address what may have caused the Big Bang itself. Atheists, like many scientists, might admit that our understanding of the origins of the universe is limited at this time. However, they would likely emphasize the importance of scientific exploration, research, and evidence-based explanations rather than resorting to supernatural or religious claims to fill the gaps in knowledge.
It's important to note that atheism is a broad term encompassing a range of perspectives, and individual atheists may have different responses or beliefs. These are general considerations often raised by atheists when discussing the necessary being argument or the cause of the Big Bang.