There are some things that point to an existance of religion(s). Take Udyr and Lee Sin's lore, for example. Udyr was taken in by monks and Lee Sin is a monk. To me at least the word monk implies serving for one or more gods. Please correct me if I am wrong.

I have heard religion defined as a way of life before, so maybe these monks just follow a diferent path of life significant enough for them to be called monks.

I wouldn't be suprised if there was already a thread about this, so I apologize if this is redundant.

There was an old Red post by Runaan or someone about this, there are several religions and gods/godesses in Valoran.

Well Nasus is a God

He seems to be more like a mortal, symbolic avatar, rather than a god.

You are thinking about the christian monks, who devote their lives to studying biblical teachings. But I think the Ionian monks are based on Buddhist monks, who devote their lives to attaining inner peace (among other things). Buddhists, general, don't worship any gods, so I suppose Ionians don't as well.

Malzahar leads a cult following in the worship of the void creatures that keep talking to him, while Kassidin leads a cult following in the service of stopping the void creatures from taking over.

Leona belongs to a group of sun worshipers.

Amumu has been (obviously) mummified, meaning his ancient culture likely believed in an afterlife.

Renekton and Nasus may resemble Egyptian gods, but they didn't seem to have taken up the roles of gods that would expect humans to worship them.

I'm sure I missed a bunch of things, but I hope that helps for starters.

I got some more!

Soraka apparently basically was a transcendent being that could tap into the magic of the cosmos. But apparently even such a being can fall down for indulging their anger. Probably a good thing considering the Greek gods and their stories...

Kayle and Morgana are also something beyond far mortals and their angelic appearance obviously raises questions, though whether they are spiritual top-dogs or part of something greater or even neither is unknown.

Anivia's apparently something special as she is an immortal being formed out of the eternal essence of her home world. If she isn't a goddess, her story is nonetheless very much befitting of one.

Janna's power is apparently one of "an avatar of air", and is born from a primal force of nature. It even changed her appearance. While nature isn't technically a religious thing, I'd say what happened to Janna implies the wind isn't just a bunch of blowing air when you mix magic into consideration - and otherwordly things aren't completely unlike spirituality.

The Rakkor are stated to revere combat and war as the ultimate artforms - while such a thing is likely to influence their culture's spiritual tendencies, it is unknown if that is the case or whether the concepts just hold much of their attention and prestige. After all, as stated, the Solari Leona is a part of instead is a clear religion.

Maokai's judgement implies his saplings are created from the dead while Yorick's background states death has a many-tiered ladder that spirits can ascend. Both make it clear it that you may not simply stop existing when you die in Runeterra.

I think that should be it for now for all that can be inferred on the spiritual and religious side of things in League of Legends. Obviously, the universe hasn't been absolutely explicit about the topic thus far aside from the Solari - perhaps despite the statement of Runaan mentioned above, the overall cultures of most of the city-states are without religious influence and religion tends to be more separated within their groups? It seems like a logical conclusion for many of the city-states: As militant nationalists, Demacia and Noxus would probably prefer their culture to stay devoted to the city-state's ideals instead of any others. With Piltover's scientific endeavours, it would be a logical guess its people overall prefer to not assume anything about the world's working. Or maybe the magic of the world quenches most people's longings for explanation on the how the universe works?