Montana Wildhack wears a locket on which is written, "God grant me the serenity to accept the things I cannot change, courage to change the things I can, and wisdom always to tell the difference" (9.33.21). The same words appear framed on Billy's optometry office wall in Chapter 3, Section 12. We find this prayer really striking for two reasons.
First, the prayer appears in both Billy's real life and his Tralfamadorian life, strongly hinting that his Tralfamadorian experiences are made up. He has taken bits and pieces from things he has seen in his daily life and read in science fiction novels to make up a world he wants to live in.
Second, this prayer expresses something profound that Billy is really looking for. He does want to find a way to accept what he cannot change (the past), the courage to change what he can (his current reality), and the wisdom to tell the difference. In a sense, Tralfamadore is all about granting this prayer: the Tralfamadorian belief in unchanging time means nothing can be changed, so there's Billy's serenity right there. And Billy's abduction gives him a perspective on his reality, which allows him to find the courage to tell the world about his new philosophy – to try to change humanity's sorrow and pain over death. It's pretty brave to try to change the world.
Note that even though this is a prayer, it's the Tralfamadorians that grant Billy this new serenity and courage, not God. God in the novel is strongly associated with the pain of death. Billy's mother's crucifix focuses on the agony of the crucifixion, and Kilgore Trout's two novels about Christ both emphasize his death and its purpose. Billy has seen enough pain and suffering among the innocent in his own life; he doesn't need Christianity to give him new examples. Instead of turning to the suffering Christ, Billy looks to space aliens for relief from his misery.