Symbolism, Imagery, Allegory
The cross is the symbol of the church. It's the symbol of God. As Barlow says, it's "the symbol of White" (14.277). Matt uses a cross to frighten away Mike Ryerson; Ben uses a cross made out of tongue depressors to drive off Mrs. Glick. Father Callahan's cross destroys the lock on the Marsten House, and then holds Barlow himself at bay—for a while.
Symbol of the Church
The cross as symbol of good is not just what the cross symbolizes in the book; it's also what the cross symbolizes in the church. Vampire legends draw on the church's symbolism, and King in turn draws on the legends. As a result, the cross means exactly what the cross means: it's a symbol of God.
Sort of. The novel does use the cross, but it's cautious, or maybe outright confused at times, about that symbol's exact relationship to Catholic theology. Barlow writes that "the Catholic Church is not the oldest of my opponents" (14.276).
Barlow's not exactly trustworthy, but he certainly seems to be saying that Catholicism is just one kind of magic he's fought, one that works on him but isn't the be-all and end-all. Similarly, when Ben is in the confessional, he thinks of the Church not as a conduit to God, but as "a direct pipeline to the days when werewolves and incubi and witches were an accepted part of the outer darkness and the church the only beacon of light" (14.191).
Symbol of Horror Books
You might even say that the cross symbolizes faith in monster stories. After all, it's a charm whose powers come not so much from the church, but from King's own Bible, the monster movies and stories he loves. That's why Father Callahan isn't the main hero of the novel: his cross, the church's cross, isn't the right one to totally fend of the monsters.
Instead, it's Mark, who pulls his plastic cross from the "plastic graveyard" of his monster set who defeats first Danny Glick and then (with Ben) Barlow (10.412). The cross in this book, which glows and brings down blasts from above, doesn't only or necessarily symbolize faith God. It symbolizes faith in the very pulp horror fiction you're reading.