Bad arguments against the existence of God

Bad arguments against the existence of God are those that most theists will easily rationalize their way out of or that are frequently misused by atheists. These may be logical fallacies, or paradoxes that theists can explain away.

Omniscience Versus Free Will
Some atheists will argue that since God supposedly knows everything, then everything is predestined. Since many theists do not accept the predestination doctrine, this appears to be a dilemma: either they must accept predestination or God is not omniscient.

Apologists who do not accept predestination may get out of this problem by arguing that God simply knows every possible outcome of every possible decision anyone can make, thus resolving the dilemma.

God allegedly knows every possible outcome. God does not know which outcome actually will happen Therefore God is not omniscient.

Apologists who do believe in predestination may disregard free will entirely.

This argument can easily become a straw man argument. It attacks the theist's position based on the assumption that he holds to a single, strict definition of omniscience. In reality, many resolve the paradox by:
 * separating total omniscience (knowing absolutely everything) from inherent omniscience (having the ability to know anything one chooses to know), and attributing the latter to God.
 * defining omniscience as knowing all things that can be known naturally (all things and events past and present). "Knowledge" of the future is limited to prediction and planned intervention.

Not wanting there to be a god / Hating God
In The Last Word, Thomas Nagel wrote: "It isn't just that I don't believe in God and, naturally, hope that I'm right in my belief. It's that I hope there is no God! I don't want there to be a God; I don't want the universe to be like that."

This is not a valid reason for disbelief: God either exists or doesn't exist, regardless of what we want.

Similarly, maltheism - the idea that god is evil - requires the existence of god. 1. I hate God 2. Therefore, God does not exist

This argument is invalid due to the contradictory nature of hating something that does not exist.

Religion as conspiracy
Films such as Zeitgeist have led some atheists to believe that religion is some sort of conspiratorial effort to control people. While there is an argument to be made that religion does control people, it is certainly not the case that religious authorities in general are actively, cynically manipulating people for their own ends. Sometimes, atheists arguing this point will quote Karl Marx out of context as saying that "Religion is the opium of the people." This is meant to imply that leaders use religion as a method of keeping the people complacent. In fact, Marx was not implying this at all. The quote in context reads:

"Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people. The abolition of religion as the illusory happiness of the people is the demand for their real happiness."

Marx is saying that religion gives people false hope, not that it fosters complacency. Marx makes a good argument here, but it is not the argument many people think it is. On a practical note, using arguments put forth by a Communist should be done sparingly, as many theists will jump at the opportunity to use an ad hominem attack accusing atheists of being Communists.

Quantum Mechanics, String Theory, and Relativity
While there are legitimate arguments to be made using these physics concepts, they should only be made by people who are very well-versed in the science. Most people are not familiar enough with these concepts to use the arguments effectively. Furthermore, lay audiences will almost certainly not be familiar enough with the science to follow the arguments very well. This means that the audience will either lose interest in the discussion or be naively persuaded by the smoother talker, even if they're wrong.