Symbolism, Imagery, Allegory
When Carla and Deborah walk out into the spring rain on a Sunday beyond the boundaries of where they're allowed to go, they both feel good and free and normal. Their experience is described as if it were a baptism.
Baptism is a traditional ritual in the Christian tradition that reenacts the way John the Baptist dunked Jesus in the River Jordan. Christians carry on this tradition to this day in order to inaugurate someone into the religion. Sometimes a pastor or priest dunks you in water and blesses you with some prayers; sometimes you get just a sprinkle of water over the forehead. But it always involves water.
Baptism symbolizes the washing away of the original sin of Adam and Eve in the Book of Genesis in the Bible. Their sin? They didn't follow the rules God set for them—so God threw them out of Eden.
Not listening to the rules in the Garden of Eden, disobeying God, and being thrown out of paradise and doomed to die. So anything described in a work of literature that sounds like baptism in some way is a big deal. It's about being forgiven and going forward with a renewed sense of purpose and commitment to being a good person.
Deborah and Carla dance around in what they call the "Sunday God's Rain" (26.22). They break the rules and have fun together as friends. After the incident, they both get the courage to leave the hospital and try for a life in the real world.
It's as if they got baptized and got some extra joy juice from the big guy upstairs, and it gives them the extra push they needed to believe in themselves, get healthier, and go check out the real world.