What is the statement 'God is Dead' talking about, and how is it relevant to the rise in Atheism?

Nietszche came up with the phrase 'God is Dead' in his book The Gay Science. By saying God is dead he is directly relating to the rise in Atheism and how more and more people were starting to become dissolusioned with christianity. In his book Nietzsche refers to a madman shouting in the streets that God is dead which shows how athiests were viewed in the time that he was writing. Nietzsche also suggests that humans have killed christian morals. He claims this as there was a rise in the belief of science which does not come with integrated morals. Here he is also suggesting that old truths that were held by christians are losing validity due to science displacing them. Due to Nietzsche and science a rise in moral nihilism can be seen and this is directly linked to the phrase God is dead and the rise in atheism.