So Cracked.com wrote an interesting article...

[Elly]

funny how i figured Fal'Cie were demi-gods of a sort and l'Cie were the agents of said demi-gods... it's honestly not as hard to figure out as the detractors exaggerate it to be... it does not take the datalog to figure it out, it's all there in story context as long as you pay attention... i got it all on my first playthrough... as for that paragraph he wanted explained, here goes:

XIII begins in Cocoon (an artificial satellite), where citizens of Bodhum (a seaside town within Cocoon)​ are being murdered by PSICOM, soldiers of the world government, for coming in contact with a demi-god from Pulse (the Planet they orbit). The demi-gods are turning people into their reluctant agents, who have an Unclear Mission, but if the agents fail to complete their unclear mission they're turned into horrible monsters. Our hero, Lightning, teams up with members of NORA (a local resistance group) to rescue her reluctant agent sister from the demi-god Anima, and the story continues naturally from there.

it's really that easy to get, and i honestly believe if they worded it that way they would have been panned for making the story over-simplified...