The purpose of the allusion in Rebecca's quote in Act IV of The Crucible is to show resistance in the face of a corrupt authority.
Arthur Miller was attracted to the subject of the Salem Witch Trials because of its spirit of resistance. While most focus on the hysteria, Miller believes that the event's real value lies in those who died for their convictions:
In Salem you have the story of a defeat because these people were destroyed, and this makes it real to us today because we believe in defeat. But they understood at the same time what was happening to them. They knew why they struggled... they knew how to struggle... they did not die helplessly. The moral size of these people drew me... they didn't whimper.
It is the resistance of those who refused to "sign their name to lies" that attracted Miller. This spirit of conviction can be found in Rebecca's last words.
The full force of Miller's meaning can be seen in Rebecca's allusion to the divine. She alludes to the power of God. She stands true to her convictions until the very end because of her faith in the divine. The allusion's purpose is to show her religious faith. Even in the face of death, she is able to stand defiant because of her faith in the divine. The allusion to a higher force is her way of indicating that those in the position of Salem are not as powerful as they believe themselves to be. When she tells Proctor, "Let you fear nothing! Another judgment waits us all," Rebecca Nurse is issuing a challenge. She is challenging Salem's figures of supposed authority to take responsibility for their actions. Men like Danforth and Parris will have to account for their actions in front of another judge: God.