Here are four core arguments against God's existence that have persisted since ancient times. Using these four arguments as an organizing principle, I present for your consideration the Four Horsemen of 1833-1856.
The argument from reasonable nonbelief- If the existence of the gods were self-evident, everyone would believe. The burden of proof is on religion. As Heinrich Heine observed, faith is not a virtue.
The argument from evil- If the gods are real, they have a lot of explaining to do. Charles Darwin could not imagine a creator with cruel intentions.
Lack of empirical evidence- Discovery trumps revelation. This is the skeptical or naturalistic argument. As Elisabeth Cady Stanton observed, revelation is often in conflict with evidence.