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 Enlightenment Horsemen.
The argument from evil- If the gods are real, they have a lot of explaining to do. Voltaire saw God as a mythical despot.
Lack of empirical evidence- Discovery trumps revelation. This is the skeptical or naturalistic argument. Hume knew that miracles were extraordinary claims that required extraordinary evidence.
The argument from inconsistent revelations- Sectarian religions are mutually contradictory and internally inconsistent. Jefferson predicted that Christianity would eventually go the way of all other obsolete religions.
The argument from reasonable nonbelief- If the existence of the gods were self-evident, everyone would believe. The burden of proof is on religion. Madison understood that faith is not a virtue.