As far as I can discover from interviews and from books, there are at least 21 reasons smart people find God unconvincing. Here are the 21 reasons, explained:
1. God as an old white man in the sky is unconvincing
Depictions of God as a humanoid (this is called 'anthropo-morphism' = in human form) have been considered incredible since ancient times. Rendering God as a male humanoid is not credible to skeptics. Does this male God have genitals, a deep voice, facial hair? Also, it's curious that God usually resembles whoever it is that's depicting God--and that's men! Men made God in their own image and likeness.
2. God as immaterial and yet with biological functions is unconvincing
It's self-contradictory to say God is 'immaterial' and in the same breath say God sees, hears, speaks, and feels--all of which are functions of biological, material organisms. What does God 'see' with if not a material eye? 'Hear' with if not a material ear? 'Speak' with without a material mouth? These descriptions of God are self-contradictory and nonsensical.
3. God as all-knowing is unconvincing
Does God know terror, fear, pain, indigestion, menstruation, sexual attraction, sexual climax, pregnancy? Does God know the last number? No one is all knowing. Again, a nonsensical claim.
4. God as all-good is unconvincing
God is all-good and yet made a world that is bloody red in tooth and claw? God is all good and made predators and prey? (A lion might love the arrangement, but the hapless gazelle and bunny rabbit do not.) God is all-good but made the talon and the fang? God is all good but made 50,000 pathogens that are trying to kill us and the animals? God is all good and made diseases? What about all the suffering of animals and humans? In human affairs, would we call a person 'good' who sits by as a six year old boy beats a two year old girl and her pet kitten to death? No, any good person would intervene. And yet God has allowed a near infinity of pain and suffering to exist. Either God is not all good (or not all powerful) or God does not exist: these are the only explanations for the high degree of animal and human suffering on earth. More likely, there's no God.
5. God as all-powerful is unconvincing
In human affairs, would we call a person 'powerfully strong' who sits by as a six year old boy beats a two year old girl and her pet kitten to death? No, if you are powerfully strong (and decent), you stop the boy killer. And yet the all-powerful God has allowed a near infinity of animal and human pain and suffering to exist. Either God is not all powerful (or not all good) or God does not exist: these are the only explanations for the high degree of animal and human suffering on earth. Also, to point out the contradiction of saying anyone possesses 'all-power,' skeptics ask: Can God make a rock so heavy that God can't lift it? Again, 'all-power' is a nonsensical claim.
6. Personal testimonies of God are unconvincing
People who testify that they experience God in some way are found in every religion under the sun during the entire span of human history. Does the personal testimony of an ancient polytheist convince you all those ancient Gods existed? Does the personal testimony of a Voodooist convince you of the truth of Voodoo? Does the personal testimony of a Caodaist convince you of the truth of Caodai? All these people have had 'an experience' (a sensation or a special feeling) but it wasn't God they experienced: it was a 'feeling' triggered by the very idea of God.
7. An 'intuition' that God exists is unconvincing
'I just know in my heart of hearts that God exists' can be used to prove anything, such as: 'I just know in my heart of hearts that God does not exist.' Or: 'I just know in my heart of hearts that many Gods exist.' Or: 'I just know in my heart of hearts that an invisible pink teacup orbits the moon.' All these are valid if any one of them is valid.
8. Answered prayers are unconvincing
Do the answered prayers of all the various religions prove to you that those religions are true? Ancient people had their prayers answered by Zeus. More likely, things just go a petitioner's way every now and then. Also, there are billions and billions more unanswered prayers than answered prayers: should unanswered prayers count against God?
9. Religious virtuosos (saints, prophets, mystics) are unconvincing
They disagree with each other about what they experienced. And they disagree about God. They are in all religions.
10. Miracles are unconvincing
ALL ancient religions are bulging with miracle stories. Are ALL ancient religions therefore true that have miracle stories? Do you believe in many Gods because polytheistic religions have many miracle stories? Miracles are fictions in all religions. As to present-day claims that God heals bodily diseases, why doesn't God heal amputations by growing limbs back? Is that too hard for God to do? Why don't we see discarded prosthetic limbs alongside discarded crutches at 'faith healing' events?
11. Revealed holy books are unconvincing
None of the books are conspicuously divine. Human talent can explain them all. People from one religion do not acknowledge the revealed claims of another religion's holy book, and skeptics simply doubt them all. Plus, some of the material in any ancient holy book is untrue and often sub-ethical by present-day standards.
12. God is not a convincing cause of the universe
God is not a solution as to the origin of the universe but only another layer of mystery. What caused God? It's more believable that a material universe emerged from preexistent matter or energy than from a non-material Mind.
13. God is not a convincing explanation of design in nature
Natural selection explains the appearance of design in nature. Besides, if a complicated thing (nature) needs an explanation in a Designer (God), doesn't the complicated thing called 'God' also need an explanation in a Designer? Who designed the Designer God? Another greater Designer God? And who designed that Designer God?
14. God is not a convincing explanation of morality
Moral rules emerged apart from religion and came from our species' cooperative, altruistic, and inhibitive tendencies, all of which were utilized by natural selection to help our species succeed. No God ever gave humanity a moral rule. Adult humans honed moral laws through trial and error and socialized children into the rules of civilization. Children absorbed the rules as 'conscience' and grew up keeping moral rules. Religion simply came along and legitimized morality by claiming the moral rules came from the Gods. 'Guilt' attends any action that opposes the socially instilled 'conscience.' Also, non-believers in God are as moral as religious people, and some are more moral. Also, highly regarded moralists like Buddha and Aristotle, and all modern secular moralists, offer sophisticated, lofty morality without any appeal to a God.
15. The number of believers in God is not a convincing 'proof' of God
Millions, even billions, have been wrong before. In ages past, everyone was wrong about many things: the shape of the earth, the causes of disease, slavery. For thousands of years almost everyone was polytheistic.
16. That God belief has benefited the world is unconvincing
Monotheisms especially have been the animating force behind the death of millions and millions of people, and religion has had psychologically harmful effects, as for instance in the creation of false crimes like masturbation and gay sex.
17. Theists disagree too much with each other (and that makes God unconvincing)
Hundreds (indeed thousands) of theistic sects suggest no one of them is true. If there's a God and God has spoken, wouldn't the world be in convinced agreement about God? Instead, we have a cacophony of discordant voices.
18. God is not funny (and that makes God unconvincing)
No depiction of God in any religion renders God as having a sense of humor, and
humor is a very high virtue that humans prize. God supposedly has all other human
virtues to an infinite degree. We are good and God is infinitely good. We are smart
and God is infinitely smart. We are kind and God is infinitely kind. We are witty and funny, so why isn't God infinitely witty and funny? Why can't God tell the funniest joke anyone has ever heard--a joke so funny that you would literally die laughing at it?
19. God belief can be explained naturally (and that makes God unconvincing)
Modern university disciplines like anthropology, psychology, sociology, evolutionary biology, and evolutionary psychology adequately explain the rise and success of religion and God belief. In prehistoric times, simple ignorance of natural causes led people to suppose there were super-natural causes for things. And when the natural causes were eventually discovered by some new science, some part of the super-natural God dissolved. It was thought for thousands of years that a God dragged the sun across the day-time sky in a chariot. Then came astronomy and cosmology and astrophysics to explain the real reason the sun appears to move across the sky. It was thought for thousands of years that God or demons cause diseases. Then medical science and biology uncovered germs. It was thought that God painted all the rainbows. Then came meteorology and optics with the true explanation. And on and on. Psychology has credible ideas about the rise of the 'Father-Figure' God.
20. God is not a 'certainty' when many, many competent thinkers doubt God
Hundreds of millions of people on planet earth lack God belief. That is a not a trivial number of people. Tens of millions of Americans doubt God. And that's not a trivial number either. The quality of many of the minds doubting God must be noticed, because many are highly educated and some are even geniuses. For a thousand years ALL the intellectuals and geniuses (100%) in the West (probably) believed in God. Nowadays, a majority of our intellectuals and geniuses (60%? 70%?) do not believe in God (most Nobel Laureates, most top scientists.) Though their intelligence is not infallible and they could be wrong, these people are not evil and are not disbelieving as an excuse to 'disobey God' and act immorally. They have real problems believing in the idea of God, as evinced in this very list of 21 items. As Emerson said of them, their skepticisms are not gratuitous or lawless but suggest real limitations on affirmative statements about God.
21. God belief may be an early phase of human evolution
Cro-Magnon peoples thought they were at the height of the human ascendancy (and they were!) They had their well-appointed caves, their tattered clothing, their grunts, their flailing gestures, their ability to count to five. But 30,000 years have passed since Cro-Magnons and homo sapiens have become quite a bit more sophisticated since then. What will another 30,000 years bring to the human race? And another 30,000 years after that? And another 30,000 after that? You and I have not arrive late to the story of humanity: we have arrived when humanity is still in its infancy. We are fresh with morning dew. And we have much evolving to do yet. None of today's religions will survive. Religion itself won't survive either. God belief will be seen as a human-contrived AID that humans outgrew (as a child outgrows a pacifier). God belief is not innate to human beings, nor is it necessary for human well-being, and the proof of this is the millions and millions of people who do not believe in God and feel nothing is lacking in their lives without God. (The most secular nations have the most civil societies, as Scandinavian countries.) In 30,000 years, no one will believe in God, just as no one now believes in the Gods of ancient Greece and Rome.
And those are the 21 reasons smart folks find God unconvincing.