In Eastern religions we have the idea of a "dharma" or the path that you follow. So different people need different paths. In Buddhism for instance, then even the four schools of Tibetan Buddhism (one of which I follow myself) have very different approaches. Buddhists in the Therevadhan traditions of Thailand and Sri Lanka have a very different approach, and again the Chinese Buddhists, and yet another approach taken by the Zen traditions of Japan and Korea etc.
These have ideas that are mutually incompatible in detail. E.g. in Therevadhan Buddhism it's thought that you take rebirth immediately when you die. In Tibetan Buddhism they think that this is sometimes what happens but often you may spend weeks in a "bardo" state between one life and the next. You simply can't fit those two ideas into a single belief system.
In the same way - though more subtle to explain, the four traditions of Tibetan Buddhism are incompatible if treated as belief systems.
But - Buddhists aren't bothered by that. The spiritual path is not a search for a final true "belief system" for us. And indeed the Dalai Lama for instance he is lineage holder in all four schools, he follows four different incompatible belief systems you can say. But it is no problem for him. Though for many practitioners that's quite hard. In some of the Buddhist schools then you deliberately meditate through a sequence of incompatible belief systems one after another, each more refined than the previous one. It's a meditative practice to help one not be too caught up with concepts and belief systems, and to see that all our concepts are somewhat less sharp edged and more fluid, in a way, than we realize.
That's with some exceptions. There are a few "renegade" Buddhists that have the idea that theirs is the only true path for Buddhism. And others that, while not going that far, say that it is confusing to listen to more than one system of Buddhist teaching, and even though they also do meditations to help them see the fluidity of concepts - they encourage their followers to only listen to the teachings of their own school. They seem to think that for the members of their own school, that their own way of relating to the fluidity of concepts is better than any other way of doing it....
But - both of those would seem to be going against the Buddha's own advice, to be open to receiving teachings and learning from anyone. And not to accept anything on the authority of others, as a final answer, but to examine everything for yourself as best you can along the path.
Anyway - if you have the idea that yours is the only true belief system then that makes it rather hard to find room in the world for other belief systems - you tend to think they have to be mistaken. Perhaps understandably so , you might say to yourself - they don't know any better - but still mistaken.
But if instead you think of it as a path you follow. And that - along the way, it is helpful for some people to think in some ways, and for others to think in other ways. And to accept that - for instance - there is no way I can absolutely decide what happens when I die. How can I know if I will take rebirth instantly as the Therevadhans say, or take rebirth after some weeks, as the Tibetans say usually happens - or go to some other place such as a pure land, as pure land Buddhists expect to happen to them - or - some other idea such as the ideas of what happens after you die in many other religions? It's a mystery we can probably never know the answer to in a definitive way.
But - if you accept that - then - for ordinary beings if not yet enlightened, you could spend your entire life worrying over this and thrashing those ideas out in your mind. But instead - it seems to work far better to just find some approach and path that works for you. And in Buddhist teachings, then the particular ideas and beliefs are not really that important in themselves. Main thing for instance is to have an open mind about what happens when you die. If you have an open mind about the many things you can't know, then that's best of all. With ideas of rebirth or whatever you find easiest to relate to as things you can accept as a kind of temporary interim truth until whenever you can see things more clearly for yourself. Just basically as a way to have something to think about so you don't spend your whole life worrying about it.
If you take a more Zen like approach, then you don't even need to do that. But just relate directly to whatever you can know for sure, as best you can. But that's a difficult path for many.
So - same way - other religions - they are the paths for other practitioners. So I think the dharmic religions such as Hinduism and Jains - they all have this idea of the dharma as the path you follow, I think helps to make you more open to having a multitude of different religions co-existing.
And - I think also in other religions many do have this idea that you can't absolutely say that your religion is right and others are mistaken. But in a way I think it's not even a big deal if you do think that. Like when friends totally disagree on something but they remain friends and respect each other and maybe even enjoy their differences of opinion and the lively debates. The main thing that helps is if you have mutual respect.
Anyway one way and another, you also get much by way of interfaith dialog with other religions also. Many dialogs e.g. between the Buddhists and the American Indians, or Christians. And they learn a lot from each other.
Here for instance is a picture of the Dalai Lama meeting with the "thirteen indigenous grandmothers" from various indigenous peoples such as Papua New Guinea
When you have mutual respect like this, you can all learn from each other. But not in the sense that the Dalai Lama is about to become a Hopi Elder or the Hopi Elders about to become Buddhists. Or that any of them are going to follow the spiritual path of the 13 indigenous grandmothers - or indeed that any of those are going to follow the spiritual path of any of the others.
All are fine with the path they follow. And their belief systems can't be mushed together to make a new religion that combines them all together. Differences are good and need to be appreciated.
So - I think this will continue into the foreseeable future that we will have many belief systems for different beings. And it will be interesting if we meet ETIs. I think they also may well have many different belief systems and spiritual paths and dharmas that they follow also.
Sometimes you get the idea that ETs would have solved all our technological problems, and have immensely long lives, so why would they care about anything? Why would they need a religion or spiritual path?
But they would still, however long their lives are, still have the possibility of accidents. And eventually they would die. And even if they have such immensely long lives due to their advanced technology that they can expect to live for at least trillions of years, so the universe hasn't yet been around long enough for any of them to reach old age or to die - then -if nothing else, then - maybe after you reach your billionth birthday you start facing feelings of boredom, even if you are a super intelligent or very happy ET? Or in some ways their lives may be less than totally satisfactory for them. Then they will be searching just as we are for ways to deal with those issues. Even if they seem to us, when we first encounter them, to have solved everything. After all many in the world look to the rich countries in the West and our lives seem like a sort of paradise to them, because we have just about all our material needs, at least the immediate ones, all sorted, or so it seems. But we have many problems in the West.
In the same way, I think that ETs would have many problems so would have spiritual paths to follow as well. And probably like us would have many different paths for different beings. And again - like us - I wouldn't be surprised if they also haven't got a final answer to many questions such as what happens when you die.
And probably the ETs might well have things to learn from us as well as vice versa. Perhaps less so in the case of technology and maths, maybe if they have a billions years old civilization there may not be a huge amount they can learn from us (on the other hand there could be -ancient civilization needn't be a continually growing one, they could stagnate and there's the intriguing possibility that ETs might have completely different maths from us, see Is it possible that an alien civilization has completely different mathematics than ours? Is mathematics absolute?)
But whatever the situation for maths or science - I think that they might well be on more equal terms with us when it comes to spiritual paths. Just as we can continue to be inspired by spiritual teachers who taught thousands of years ago before there was science or maths in its modern sense, I think also that ETs may well be inspired by spiritual teachings even from billions of years ago, which are still new and relevant to them today. And because in some ways technology hardly changes anything here, it might be something we can share with them and vice versa. Not that we would become ET religion junkies, but that we might have much to learn from and mutually appreciate spiritually.
Where of course I mean religion in a very general way. Any kind of spiritual teachings, dealing with issues beyond those of just being able to fend for yourself materially. Whatever those issues are even if they are just issues of boredom from an enormously long and happy life.