So I've seen some quotes from various Romans, questioning the reality of gods as they understood them.
Religion is regarded by the common people as true, by the wise as false, and by the rulers as useful.
Live a good life. If there are gods and they are just, then they will not care how devout you have been, but will welcome you based on the virtues you have lived by. If there are gods, but unjust, then you should not want to worship them. If there are no gods, then you will be gone, but will have lived a noble life that will live on in the memories of your loved ones.
If horses had gods, they would look like horses
But, it is also my understanding that religion in ancient Rome was more about performing worship rather than believing the correct things. That, the stories and lineages of the gods, what exactly their natures and powers were, and any ambiguities or contradictions of those claims were not a concern to ancient Romans. But, if one refused to participate in public rites, such as sacrifices and ensuing meals, that was seen as anti-social, an action against the state, and would threaten to bring disaster and downfall to the people.
So if a Roman philosopher questioned or disbelieved in the gods, what impact would that really have? Would they still participate in the acts of worship? Did any of these philosophers refuse to participate in them, because of their (dis)beliefs? Or were questions or expressions of atheism a more-or-less harmless philosophical exercise/position?
I want to start by saying the premise is wrong, as none of these authors are "atheists" nor are these examples of atheism. Be careful when seeing quotes on the internet, especially for ancient texts that require translation and thus can be heavily misinterpreted/mistranslated. Properly cited ancient quotes will have the work, a book/chapter/line citation, and a translator. Before getting to your question, I need to take issue with these quotes.
- As far as I can tell, Seneca the Younger never wrote this or anything close to it. It may have derived from a similar statement in Gibbon's Decline and Fall of the Roman Empire, but otherwise it seems to be a popular internet quote with no basis.
- I think this Marcus Aurelius quote is coming from his Meditations 2.11, but is taken out of context or mistranslated. Here is a translation adapted from the 1916 Loeb Classical Library translation by C.R. Haines:
Let your every deed and word and thought be those of a man who can depart from life this moment. But to go away from among men, if there are gods, is nothing dreadful; for they would not involve you in evil. But if indeed there are no gods, or if they do not concern themselves with the affairs of men, what's it to me to live in a universe empty of gods or empty of providence? Nay, but there are gods, and they do concern themselves with human things; and they have put it wholly in man’s power not to fall into evils that are truly such. (my emphasis)
Marcus Aurelius is discussing Stoic ideas surrounding not fearing death; his argument is that the gods would not let anything bad happen to humans. He is not saying there aren't gods, in fact he explicitly says the opposite.
- The same thing is happening with this Xenophanes quote; it is not exactly what he wrote. Xenophanes only exists in fragments; this one is quoted from Clement of Alexandria, Stromata 5.109.1. The full English from the 2016 LCL translation by Andre Laks and Glenn Most:
But if oxen, <horses> or lions had hands or could draw with their hands and create works like men, then horses would draw the shapes of gods like horses, and oxen like oxen, and they would make the same kinds of bodies as each one possessed its own bodily frame.
This is linked with another fragment where Xenophanes describes how different races depict gods with their own features. It is about depicting gods in art and about human assumptions, not about their existence. Since it is only a fragment, there isn't much more context, and being quoted by a Christian author could potentially muddle it further.
So, none of these authors are quotes are really "atheist" or expressing views that the gods don't exist. Back to your question, you are correct that action was more important than "belief" in the ancient Mediterranean. Atheism would be quite alien to ancient people because it is so tied to belief, whereas ancient practices were not. Most scholars of ancient religion today avoid belief and some have debated whether "religion" itself is a category that can accurately describe the ancient world because as a concept it is heavily tied to Protestant Christianity. Essentially, philosophers might have been able to muse over the existence of gods, but they didn't necessarily take up an atheistic "belief" over this and did not change practices. Even if they did, it would be a small individual act and most people wouldn't care or notice. I recommend Brent Nongbri's Before Religion: A History of a Modern Concept for more on religious theory and the ancient world.
I see now that u/LuckyOwl14 has examined your quotes and recommended scholarship on the topic, great work! I can only note about the first quote, that it comes from Gibbon's Decline and Fall of the Roman Empire. In Volume 1, Chapter 2, he writes:
The various modes of worship, which prevailed in the Roman world, were all considered by the people, as equally true; by the philosopher, as equally false; and by the magistrate, as equally useful.
The folks over at Wikiquote traced its attribution to Seneca to American writer and anarchist thinker Elbert Hubbard, who claimed Gibbon was "making a free translation" from Seneca (though none has found anything like it in his works).
Below I can also link to some more answers about atheism in Antiquity from flaired users: