What does Genji mean when he says that the Buddha is trying to teach him that the world
slips away?

Genji Shimada shouts "Ryujin no ken wo kurae!" when he uses his ultimate, which roughly translates to "Take my Dragonblade!". Hanzo Shimada shouts "Ryuu ga waga teki wo kurau!" when he uses his ultimate, which roughly translates to "Let the dragon consume my enemies!".      

Explanation:

the catholic church withhold people's access to the bible during the middle ages.

people could go to church and listen to the teachings, but it was absolutely forbidden to own and/or read the books of both the old and new testaments. it was also prohibited to translate the books (which where originally in biblical hebrew) to any other language.

this was done so the church could remain in control of what their teachings were and to enhance their importance and power, while also being able to judge people's actions through information that was unavailable for the general public.