Why is God Necessary For:
Knowledge: Unless one knows everything, or has revelation from someone (God) who does, something we don't know could contradict what we think we know.
Truth: If our thoughts are the mere by-products of the electrochemical processes in our evolved brains, you would not get "truth" you would get "brain-fizz." Chemicals do not produce "truth" they just react. As Doug Wilson said, it would be like shaking up a can of Mountain Dew, and a can of Dr. Pepper, opening them, and watching them fizz. Neither fizz is "true," they just are. For truth you need someone (God) who transcends the natural realm.
Universal, immaterial, unchanging logic: For universal, immaterial, unchanging logic, you need someone (God) who is universal (Psalm 90:2), not made of matter (John 4:24) and unchanging (Malachi 3:6). Without God, who has universal knowledge, we could not know anything to be universally true. Without God, who is Spirit (not made of matter), we could not make sense of immaterial things. Without God who is unchanging (and logic is a reflection of the way He thinks), we would have no basis for expecting logic not to change.