Lyric discussion by J___P 

Cover art for Mayfly lyrics by Belle & Sebastian

i interpret the bible line that its a pity that the people around you have lost the meaning of what the bible really is like her diary it holds the truth of life(her life in this case). Faith is not something to pity people for having.

Let's not look too far into the religious significance of this line. I think he's using the Bible to compare her diary to because her diary is long, and it contradicts itself if you don't look at the recorded events and rules to live by from the past in the context in which they were written.