At the start of the book Perry hasn't gone to church in years, but as it goes on he starts praying, and eventually actually going to mass. And the amount of senseless death he encounters really begs the question: Why? Why now, of all times?
Two words: desperation, comfort. Perry will do anything, including pray, to decrease his chances of getting killed. But also, like Father Santora convinces him, it makes him feel comforted to go to a religious service. Maybe worshipping with others reminds him that he isn't in the war alone: everyone else wants the same thing he does.
And really, in a situation where he has almost no control over his life, anything Perry can do to improve it is a good thing.
Questions About Religion
How does Peewee feel about prayer and religion?
How do Perry's feelings about religion change throughout the story?
Why does Perry keep praying, even though he knows his enemies must pray too?
Why did Lieutenant Carroll lose some of his connection to religion?
Chew on This
In Fallen Angels, the function of prayer changes from being a way to acknowledge the dead to an active way for the living to seek comfort.
Father Santora's honesty about his own feelings and about the effectiveness of prayer is what ultimately convinces Perry to try religion.