It has been argued by unbelievers that it cannot be scientifically proven that God exists.
God defies science. (See my prior post Give Me A Reason.)
A well versed Atheist would argue that God does not exist by comparing Him to a Leprechaun. How can you prove that Leprechauns exist? That is a silly notion to say that Leprechauns exist… therefore, it can be said that God is a silly notion.