The outcome is only uncertain for those who disbelieve.

George R. R. Martin:

“The peach represents… Well… It’s pleasure. It’s… tasting the juices of life. Stannis is a very marshal man concerned with his duty, and with that peach Renly says: “Smell the roses”, because Stannis is always concerned with his duty and honor, in what he should be doing and he never really stops to taste the fruit. Renly wants him to taste the fruit but it’s lost. I wish that scene had been included in the TV series because for me that peach was important, but it wasn’t possible.”