I think it's important for a person to define or characterize what is meant by "God" as a prelude to an argument as to God's existence or non-existence. I also think many scientists who say they believe in God may have in mind Chaz's "vague impersonal organising principle of the Universe". This is not necessarily true of all scientists who say they believe in God, however. According to the book, "The Language of God" by Francis Collins, who led the successful Human Genome Project and argues for evolution, and who is also a Christian, God exists and uses evolution to bring about earth life forms.As god can mean anything from a white guy in the clouds who cares about what we eat, and who we sleep with TO a vague impersonal organising principle of the Universe, it does not mean much to offer numbers of scientists that are supposed to believe in god. God is a vague and diffuse concept.
I think the God whose existence is denied by most atheists we read about is the entity characterized as the supreme, supernatural Creator of the Universe, who loves humanity (homo sapiens), and who has tremendous (if not infinite)knowledge and power. For many folks, if not most, the existence of this God is probably easier to doubt than to believe in.
If you define God as "Love", or "The Best and most Worthy of Human Behavior", you have a different set of arguments as to whether God exists or not. If you define God as everything that is, whether understood or known by humanity or not, then barring arguments about "Is-ness", we might all have to agree that God exists.