Before the Great Awakening of the 1730s and 40s, orthodox Puritan belief about predestination was that salvation could not be earned; it was God's privilege to determine who would be among the elect and who would be consigned to eternal damnation. Puritans could only continually search themselves for outward signs to try to ascertain what their fate would be.
In "The Crucible," Rebecca Nurse seems to be a character with unassailable belief that she was among the elect. With no demonstrated sins to count, she approaches her execution calmly, secure in the belief that she would soon be with God. In this sense, she is above the fray of those struggling in the trials. One could view Giles Corey's refusal to validate the trials by entering a plea in the same way. Giles is simply beyond the squabbles of Salem.
John Proctor, on the other hand, is a Puritan who does not believe himself to be among the elect, thus it is understandable why he cherishes his earthly life and initially agrees to confess. He has no hope of salvation and so takes the only moral stand left to him: to communicate his disdain for the corruption of the lies and delusions of the trials. Proctor enacts a measure of personal redemption by not denouncing his faith in something higher than the broken theocracy of Salem.