Oh, I'm
sort of an avid player of the game, having played for almost 2 years and still do. I'll try to cover the more important subjects as there is a LOT of stoyline to the game.
Mabinogi is loosely based on the
Mabinogion, specifically, its mythological setting. There's several deities throughout the game and they have somewhat of a hierarchy system. They will interact with you and at times, give you their power or attempt to destroy you. You may even have to fight one. There's no real worship of said deities except for one, Lymilark, the deity of Love. There's several Churches set out in the world of Erinn devoted to Lymilark that serve as volunteering stations and in return you get Holy Water of Lymilark which you bless your weapons with to reduce durability loss. That's about it for that.
Spirit weapons are exactly what they are. They're weapons with spirits in them. No one really knows where said spirits come from but they are mined out in spirit stone forms and later imbued into your weapon of choice. There isn't really too much thought put into them spiritually wise. They're just very strong weapon that, when trained well enough, can release a special attack.
There's also transformations and for humans, depending on if you decide to do Morrighan the major goddess's will or go against her, you can transform into a Paladin or a Dark Knight. These are limited periods of time where you gain a major buff in stats. Giants and Elves also have transformations but there really isn't too much known about the back story of elves and giants.
I mentioned earlier that gods may sometimes give you their power. At a certain point and if you succeed, you will gain the Demi God transformation, which is separate but similar to race transformations. During this transformation, you become a half god, and are extremely powerful. Here's an
elf using it.
Hm. There's a lot of mythology and story to the game, the only other possible "objectionable" content would be
Claimh Solas and Glas. They're rather demonic-looking but they are
not demons. Again, there's not much objectionable about the game if you remember that all of it is fantasy.
Otherwise! Mabinogi is a fantastic game. You don't always have to fight all the time, you can create your own songs and play them, pick up tailoring to make clothing, or become a blacksmith to create some awesome weapons and armor! There are no classes in Mabinogi, what you train and do is up to you, there are no limits whatsoever. I'm also extremely fond of the combat system. There's no mindless spam clicking, you will have to strategically use your skills or you could die to even a rat!
If you do decide to play, I'll be glad to help you. I play on the Mari Server and my IGN is Crimmy. I am also a giant.