Mentions on Religion in League of Legends lore?

There's the obvious Kraken Priestess with Nagakaborous, the introduction of the cult of three sisters in the Freljord, also seems to have some followers, Tryndamere's tribe worships a Tusklord god all off the top of my head.

Bilgewater has a deity known as Nagakabouros. Shurima has god-warriors known as Ascended, so its pretty much ancient Egypt in that regard. Mount Targon has the Solari, who worship the sun. The Aspects could also be viewed as religious icons of some sort. Zaun kind of worships Janna. The Freljord has Demigods. Ionia is very spiritual place so their religion(s), if they have one/any, are probably connected to that.

There's numerous religions in and around Bilgewater (such as Illaoi's), followers of Avarosa, Serylda and Lissandra, the orders that existed on the Blessed Isles (such as Yorick's), the tallymen of Kindred, Shuriman Ascended worship, Targonian celestial worship (such as Leona's), and some in Demacia (the Veiled Lady).

There's Janna and Viktor's followers in Zaun. As for cults, there's the cult of the Spider and the cult of the Void. Evelynn also has her own cult.

Before everyone else scramble over each other trying to answer your question.

Might I suggest visiting the Universe site and read through some of the content provided for yourself, its not difficult and a lot more fun to uncover these things rather than impassionately from strangers on the internet.

You'll find many, many obscure things that touch on the subject of piety, devotion and most of all spirituality. You won't find much that gives explicit detailing on the individual beliefs in Runeterra, as there are too many to ever be covered and no real "main" religions. Plus, if you were hoping for a Greek pantheon or a Christian Church parallel, then you will be disappointed as there's none of that. But there are a whole bunch of societies and cultures that were/are built around adherence to something metaphysical.

Zaun has the Glorious Evolved.

Pretty much everyone follows a faith but Noxus and Piltover

Buhru and Bilgewater: Nagakaborous ( ), The Bearded Lady colloquially in Bilgewater

Ionia: The Spirit of Ionia ( ) and Balance with nature which is more a philosophy but w/e ( )

Demacia: Veiled Lady and lots of "THE LIGHT" metaphors (maybe some Targonian Aspects like the Protector as well based on the last story)

Targon: Celestials and Aspects, Solari ( ), Rakkor ( ) and Lunari ( ). Targon is more complicated because religion is at the core of the region

Shurima: The Ascended ( ), The Great Weaver ( )

Freljord: Spirit Demi gods ( ) and The Cult of the Three ( )

The Void: and his Cult

Zaun: in the Sump, Glorious Revolution ( ), Whatever preaches

Then there is who is referenced, revered and feared in all corners of the world including Noxus but it's not really a religion so I left it out as a main faith

I leave anybody out?