One of the oldest objections to popular religious beliefs and popular forms of theism is something that should be obvious, but which is too often ignored: the gods people believe in look remarkably and disturbingly like the theists who are promoting their beliefs. Sometimes these similarities have been physical in nature, but more often they are matters of philosophy, temperament, and/or attitude. Generous people believe in a generous god, harsh people believe in a harsh god, tolerant people believe in a tolerant god, and so forth.
The first recorded example we have of this line of thinking comes from Xenophanes of Colophon, an important Greek philosopher during the early 5th century BCE. Xenophanes didn't reject the existence of gods entirely, but he did reject the existence of gods which looked suspiciously like humans and thereby cast doubt upon gods generally:
Both Homer and Hesiod have attributed to the gods
things that are shameful and a reproach among mankind:
theft, adultery, and mutual deception.
And this he held was due to the representation of the gods in human form.
But mortals suppose that gods are born,
wear their own clothes and have a voice and body.
The Ethiopians say that their gods are flat-nosed and black,
While the Thracians say that theirs have blue eyes and red hair.
Yet if cattle or horses or lions had hands and could draw,
And could sculpture like men, then the horses would draw their gods
Like horses, and cattle like cattle; and each they would shape
Bodies of gods in the likeness, each kind, of their own.
All that must be swept away along with the tales of Titans and Giants,
those 'figments of an earlier day' if social life is to be reformed.
Although Xenophanes' points are largely physical in nature, even at this time he includes explicit reference to matters of temperament and attitude: the ancient Greek gods could be thieves, adulterers, and liars not unlike the ancient Greeks themselves, not to mention the rest of humanity. Although many people may think of the ancient Greeks and the ancient Greek religion as quite remote from their own, has anything really changed?
People's gods may not have flat noses or red hair so much these days, and there certainly aren't horse- and cattle-shaped gods, but the attitudes and temperaments of peoples' gods continue to be remarkably like their own. This is surely not a coincidence. The question then becomes: is people's behavior influenced or determined by what they think their god wants, or is their image of their god influenced or determined by their own personality and attitudes?
The former certainly isn't impossible, but if that's the case then how did so many different people come to believe in the existence of so many different gods wanting so many different things? That doesn't sound readily compatible with the existence of just a single god with a specific agenda and which wants particular things, even if we accept that people do a poor job at understanding what is wanted from them.
If theists really are trying to conform their personalities, attitudes, and behaviors to what they sincerely believe their god wants, then not all theists can be right. This means that most theists are clearly wrong because either their god doesn't really exist or, if it does, then it doesn't exist in the way they imagine and it doesn't want what they imagine.
The alternative is no less discomfiting to the believer, but it may be a bit more likely: people end up believing in gods with particular desires and attitudes because these mirror the believers' own desires and attitudes. In effect, people create gods in their own images. They may start from a template in a particular tradition, but if they are part of a religious tradition like Christianity which is sufficiently diverse, then it's easy to pick and choose those scriptures, teachings, and traditions which best justify what one is already looking for.
It's a form of rationalization, but likely one that happens unconsciously basically the use of Confirmation Bias and Subjective Validation, things which happen in completely secular contexts all the time. It's not something unique to religion in any way, though its appearance in religion should be seen as especially troubling. Flaws in reasoning in a secular philosophy are to be expected because that philosophy is purely human in the first place; religion, if it's true, should be resistant to such problems.
None of this proves that no gods can or do exist and it doesn't prove that no religions are or can be true. It does, however, cast significant doubt on the truth-claims of all these religions, especially the claims made on behalf of the gods which people believe in. Although some sort of god or gods may exist, it's difficult to credit any one claim in particular as being more credible than any other when it's clear that this alleged god shares so many qualities with the believer who's trying to convince us.
Even if we try to be generous and assume that such similarities are due to people trying to conform their own attitudes and behaviors to what they think their god wants and ignore the fact that others arrive at incompatible conclusions, we should still wonder how and why people would meet with such success. If there is a god, surely it must be radically different from us and have radically different perspectives, ideas, attitudes, goals, etc. It should be different enough that people trying to imitate it shouldn't be so successful.
What this suggests is that even in the best case scenario, there's a strong likelihood that people are still attributing at least some of their own personality and behavior traits to the god they believe in. If that's true, and if everyone is doing it, then it's unlikely that any single alleged god really does exist as claimed; furthermore, it's difficult to see how any one of them is more likely to be more true than any of the others. In that case, then we don't have a sound, rational, reasonable foundation to believe any of these claims.