As Simon and Ralph journey through the jungle with the other boys in Chapter Seven in a desperate search for the beast, Simon tries to comfort his friend:
"All the same. You'll get back all right. I think so, anyway."
Simon says this to reassure Ralph about their situation. Of all the characters in the novel, Simon is probably the most perceptive; he senses or can tell that Ralph has been worrying about their situation, and because Simon values Ralph as a friend, he wants to make him feel better. Ralph, however, remains unsure about how Simon can be so sure about his future, and bitterly comments "Got a ship in your pocket?" and then he calls Simon "batty" (111).
Simon refutes this with a violent shake of his dark head, and reaffirms his original statement. Despite whatever misgivings Ralph may have about Simon's beliefs about the future, his friend's confidence in his eventual rescue does make the other boy smile. Ralph may find it odd Simon's predictions odd, but he appreciates the gesture.
Simon is the moral compass and epitome of faith in the novel. While other boys compete for supremacy over others or scramble to align themselves with a leader, Simon functions very differently. Some literary analysts go so far as to label Simon a Christ-figure, and given his behavior (feeding the "littluns" fruit from a tree) and the circumstances of his death, it's a plausible reading. Simon's words just before the quotation, "You'll get back where you came from" could have the deeper, divine meaning that upon death, we all return to the source of our creation.
At minimum, Simon is written as spiritual, often choosing solitude and communing with nature over wrangling with the others. It is reasonable to read the quotation as a declaration of faith.
It is also reasonable to read it as prescient, because "You'll get back all right. I think so, anyway," is not the same thing as "we'll get back all right."