The world of the witcher is pretty much modelled after the medieval/renaissance Europe and it is apart from the presence of magic and fantastic creatures firmly grounded in realism. The weapons and armor, monetary system, and how the realms are governed are accurate to how it was in the real world. But there is one thing (again, apart from fantasy part) that's vastly different is how many religions are there – in medieval Europe there was only Christianity – there might have been small Jewish or Muslim population here and there, but most people were Christians. In "The Witcher" though, it is way more diverse. In this post, I will briefly go through some of the faiths of the Northern Realms.
The Church of the Eternal Fire (Kościół Wiecznego Ognia) – this is probably the most popular faith in the Northern Realms. It is based around the concept of the fire that symbolises hope, survival and enlightens one's path in the darkness. The leader of the Church is the hierarch of Novigrad (currently Cyrus Engelkind Hemmelfart), who also rules the City. In Novigrad there is 19 temples and each has an altar where the fire burns. The church is responsible for witch hunts and doesn't like creatures like dopplers – when they catch one, the priests conduct exorcism over them and then cover them in clay and bake over the fire. It also has armed forces – the Order of the White Rose (Zakon Białej Róży), later renamed by its grandmaster Jakub de Aldersberg to the Order of the Flaming Rose (Zakon Płonącej Róży).
the Cult of Prophet Lebioda (Kult Proroka Lebiody) – this cult seems more like a philosophical movement than religion. It's followers read "The Good Book" ("Dobra Księga"), which is a collection of parables and Prophet Lebioda's wisdom – most of the known fragments follow the style of biblical parables, but are a bit humorous (e.g. it says that you should help the poor, but if you have a watermelon, don't give it whole, because the poor may lose his mind from such hapiness). Queen Zuleyka of Kovir is a devoted follower of Prophet Lebioda, and when her husband Esterad asks her what to do with Dijkstra's request to borrow him money for war against Nilfgaard, she cites "The Good Book", which leads the King to send his prisoners as mercenaries to strengthen Northern forces. It is possible that Prothet Lebioda was a priest of the eternal fire and made the schism from the Church after disagreeing with stuff like witch hunts, meaning that his cult might be a faction of the Eternal Fire like Lutheranism is to Christianity. That assuption comes from the fact that in "Season of the Storms" we learn that the propthet's grave is in Novigrad, the city of Eternal Fire (CDPR apparently believs this theory, because Propthet's card has the Firesworn tag). However, there are too many differences between the two to confidently say that those are two factions of the same religion.
It is pretty late as I'm writing this, so I won't cover all of the religions in this post, but let me know if you want more.