According to Edwards, what do healthy, strong members of the town foolishly believe, and why do they believe it?

The healthy strong members of the town foolishly believe that they are kept out of hell by

the good State of your bodily Constitution, your Care of your own Life, and the Means you use for your own Preservation.

They believe this because they don't fully grasp the bigger picture that there is more to life than what they see with the evidence of their own senses. A spiritual world sustains them that exists above and beneath and around the physical world. They are fools if they put their faith in their well-being solely in the material.
Edwards reminds them they are alive through the will of God, not their own efforts. If God withdrew his protection, all their good health and care of their lives would mean nothing. It would no more support them than a spider's web would, or the air would, if they were suspended in midair. Edwards states that humans are weighed down with sin and, if it were not for God's grace and providence, would quickly plunge into hell. Therefore, it is more important to attend to spiritual and religious matters, especially one's relationship with Christ, than to physical well being.