1. Creation. It is impossible for creation to create itself, because it would have to have been pre-existent before it created itself, to create itself, which is scientifically ludicrous (this is true even if they call creation “Nature”).
2. Conscience. The knowledge of right and wrong is shaped by society but is God-given (even society has moral requirements). This leaves us without excuse before God when we violate His moral Law.
3. The Commandments. Just a few moments looking at the Ten Commandments--and how God sees lust as adultery and hatred as murder, leaves us all guilty on Judgment Day, justly heading for Hell (the Commandments give us good reason to need a Savior).
And, 4. Conversion. When an atheist comes to know the Lord, the argument as to whether or not God exists is over.
(Condensed from, How to Know God Exists)