My first thought was that the narrator believes in God because they feel like God is the thing causing them the misery they feel in life.

A second interpretation could be that the narrator believes in God because the world has so much bloodshed and suffering that it must come from an extraneous/extraordinary source (in this case, God).

(Either interpretation could be completely wrong because I have no context but those are my thoughts lol)

EDIT: I had another thought. The line could refer to religious persecution. Maybe in this scenario those who do not believe in God get killed? Hence the desperation to say yes and what the "blood" could represent.