Edwards claims that nonbelievers are akin to spiders in Sinners in the Hands of an Angry God because spiders are powerless should a rock fall on them.
What Edwards is trying to say is that anything you do couldn't save you from hell if you do not have faith in God, because your actions would be as fragile as a spider's web trying to hold the weight of a rock that falls into it. Those actions, as the spider's web, would have no influence, and the spider would fall in hell.