According to Edwards, what do healthy, strong members of that town foolishly believe? Why do they think this?

According to Edwards, healthy, strong members of the town foolishly believe their own strength and abilities can save them from God's wrath.

In Edwards's opinion, those who have yet to descend to the depths of hell are only momentarily safe because of God's mercy. He accuses the members of the town of trusting their own healthy constitutions to keep them out of the fires of hell. Perhaps members of the town believe in their relative safety because they have conceivably lived what they consider to be good lives: after all, they have refrained from self-indulgent habits and have pursued every righteous course known to them.

You are kept out of Hell, but don’t see the Hand of God in it, but look at other Things, as the good State of your bodily Constitution, your Care of your own Life, and the Means you use for your own Preservation. . . if God should let you go, you would immediately sink. . . plunge into the bottomless Gulf, and your healthy Constitution, and your own Care and Prudence, and best Contrivance, and all your Righteousness, would have no more Influence to uphold you and keep you out of Hell, than a Spider’s Web would have to stop a falling Rock.

Basically, in his sermon, Edwards argues the strong and healthy members of the town should rely less on their own contrivances than on God to keep them from the fires of hell.