Just as there are arguments for God's existence, there are serious arguments against it, or for doubt. This level presents them as their best defenders would, with the main replies believers give.

The problem of evil

The oldest and most powerful challenge asks how a God who is all-powerful, all-knowing and perfectly good could allow so much suffering. David Hume, in his Dialogues Concerning Natural Religion (1779), restated questions he attributed to Epicurus: “Is he willing to prevent evil, but not able? then is he impotent. Is he able, but not willing? then is he malevolent. Is he both able and willing? whence then is evil?”

Philosophers distinguish two versions. The logical problem claims that God and evil are flatly contradictory: they cannot both exist. The evidential problem makes a more modest claim: the amount and kind of suffering makes God's existence less likely, as with a fawn dying slowly in a forest fire that no one ever sees. Many philosophers today, believers and nonbelievers alike, think the logical version has been answered, but the evidential version remains much debated. Philosophers also distinguish moral evil, caused by human choices, from natural evil, such as earthquakes and disease.

Theodicies

A theodicy is an attempt to explain why a good God would permit evil. The main ones are:

• The free will defense: genuine freedom is a great good, and free creatures can choose evil. God could not give real freedom while guaranteeing we never misuse it. The idea goes back to Augustine. Critics ask whether it explains natural evil, and whether freedom is worth such terrible costs.
• Soul-making: drawing on the early Christian writer Irenaeus, this view holds that the world is a place where character such as courage and compassion grows through real challenges. Critics reply that much suffering crushes rather than builds character.
• Skeptical theism: given the vast gap between God's knowledge and ours, we should not expect to see the reasons for every evil. Critics say this makes God's goodness impossible to assess.
• Many traditions also point to an afterlife in which suffering is redeemed or outweighed.

Divine hiddenness

A newer argument, developed by the philosopher J. L. Schellenberg in the 1990s, runs roughly as follows. A perfectly loving God would want a relationship with every person capable of one, and so would make his existence evident to sincere seekers, yet some do not believe through no fault of their own. So, the argument concludes, such a God probably does not exist. Replies include that God may stay partly hidden to preserve our freedom to respond, that belief too easily gained might not produce real love, and Pascal's older theme of a God who is partly hidden.

Hume on miracles

In An Enquiry Concerning Human Understanding (1748), Hume wrote that “a wise man proportions his belief to the evidence.” A miracle, he said, is a violation of the laws of nature, and our evidence for those laws is uniform experience. Witnesses can be mistaken, deceived or eager to believe, so testimony for a miracle is more likely mistaken than the miracle is true, unless its falsehood would be even more extraordinary. Critics reply that Hume's argument assumes in advance that miracles do not happen, and would prevent us from ever accepting a genuine surprise.

Burden of proof

Who must prove what? Some nonbelievers argue that the person making a positive claim bears the burden of proof, so atheism is simply the default lack of belief. Many theists reply that saying “there is no God” is itself a claim that needs support. The term agnostic, coined by T. H. Huxley in 1869, describes someone who holds that we do not or cannot know whether God exists.

Worked example: a conversation after a flood

After a flood destroys part of a town, two neighbors who help with the cleanup talk late into the night. Grace is a Christian; Daniel lost his childhood faith years ago.

Daniel raises the evidential problem: “I can see why people choosing to hurt each other might be the price of freedom. But no one chose this flood. Children drowned. If an all-powerful, loving God exists, why didn't he stop it?” He notes that this is natural evil, which the free will defense does not obviously cover.

Grace does not pretend to have a neat answer. She mentions soul-making: a world with stable natural laws, where water floods as well as nourishes, is a world where courage and compassion are real, and she has seen both in the town this week. She adds the skeptical theist point that she would not expect to understand all of God's reasons, and her hope that those who died are held in God's care.

Daniel replies that soul-making cannot explain why the children who died had no chance to grow, and that skeptical theism, taken far, makes it impossible to judge whether God is good at all. Grace grants that these are hard objections. For her, she says, faith is less a theory that answers every question than a trust that holds through them.

Notice several things. Both take suffering seriously; neither treats the other as foolish. The philosophical arguments are real, but in the presence of fresh grief, many believers and nonbelievers alike say the first duty is practical: to help, to listen and to mourn. The intellectual question remains open, and thoughtful people on both sides continue to wrestle with it.