How would you respond to someone who tells you they don't believe in G-d?
Everyone has their own ideas of what is and what isn't, so you have to feel your way around to see what might open them up. Mostly, when someone says that they do not believe in G-d they mean that they do not believe in an ancient old grey-bearded man who sits on a throne somewhere in heaven. If they tell me that, I tell them that I agree with them.
Then, there are those who like to say that they do not believe in a Creator. I tell them that the entire world, including the scientists believed like him for thousands of years. They laughed at the Torah for claiming that the world had been created. The scientists believed that the world has always been here.
Then, rather recently they came up with the Big Bang theory, i.e. that the Universe was created with a big bang around fifty billion years ago. They changed their minds some 50 years ago, and now say the Big Bang happened only 13 billion or so years ago. But science can change their mind whenever it wants and no one throws science away. Unlike the Torah which has never changed.
Now, since everyone agrees with the Torah, i.e. that the world has been created, and by what they call a Big Bang, and we call G-d speaking, the question has to be, how did the Torah know that the world was created? No one could have been there to witness the creation and write it down, so how was it recorded in the Torah? The story of creation in the Torah had to come from somewhere higher than creation.
As to belief in G-d, what usually works for me is to ask them if they believe in Existence. Most will say 'yes,' but some who have read Buddhist ideas might say that nothing exists. When someone says that to me I usually answer, "If you do not exist there is no sense in talking to you," which might be funny, but it doesn't solve the problem.
If they say that they do believe in existence, I ask them if they believe that all existence is one. If they agree, I ask if they believe that existence is everywhere, even in the empty space. If they agree then I tell them, "G-d's most holy Name is Existence.[i] It is not that you do not believe in G-d. You believe in G-d, but you call Him Existence, which is fine…and we call Him G-d."
[i] Yud Kay Vav Kay – The meaning of this Name is Self Existence